Somebody bodged it up, not terribly, but not good.

1. two wires under one screw on the switch, not good and not code legal. The correction for this is to remove the wire going into the cable (top middle), and connect with the other black wires in the wire nut in the lower middle of the pic since it's already pig tailed (a very short wire from the wire nut to the switch).
2. The cable coming in from the lower right is coming thru a hole that was obviously drilled in the box, altering a UL listed product. Again, not code legal.
3. The ground wire from that cable was lazily wrapped around the existing ground without a proper crimp sleeve. Obviously this cable was an "add on".
4. Having all the neutrals (white wires) tied together is common and OK. The only issue could be if one or more of them have a poor or non-existent connection.
5. I disagree that you can solve this with a non-contact voltage detector. They can be very miss-leading.

Lastly, I would suggest a multi-meter and start tracing circuits. You can do that with the power off (and must), but with a non-contact voltage detector, the power needs to be on. My recommendation, BEING ABSOLUTELY SURE THE POWER IS OFF would be to take an old extension cord you don't want anymore, cut it off about 6" from the plug, strip the wires and put a wire nut on them, creating a short. Then test resistance (ohms) between each black and white wire on the same cable. Record and mark them, maybe using a sharpie and masking tape. If some are not near infinite resistance, those cables are going to lights. At any rate... After testing for voltage again, being absolutely dog gone sure the power is off by testing the outlet with your multimeter first , plug in your your shorted out plug and check resistances on all cable/wire pairs. If one of them drops to near zero resistance, you found that circuit. Repeat until you've tested all circuits.

LASTLY, KEEP YOUR INTENTIONALLY SHORTED OUT PLUG AWAY FROM CHILDREN, STUPID ADULTS, PUT IT IN YOUR GUN SAFE OR TAKE IT APART AND THROW IT AWAY! ok, getting emphatic here, it's a useful tool, in the right hands.

EDIT: Forgot to mention to get access for the continuity checks, you'll need to undo the white wires under the red wire nut, but that shouldn't be a problem to reconnect, since it seems as if all the white wires are already connected. Next, the sloppy ground should also be corrected with a proper crimp sleeve.

Somebody bodged it up, not terribly, but not good.

1. two wires under one screw on the switch, not good and not code legal. The correction for this is to remove the wire going into the cable (top middle), and connect with the other black wires in the wire nut in the lower middle of the pic since it's already pig tailed (a very short wire from the wire nut to the switch).
2. The cable coming in from the lower right is coming thru a hole that was obviously drilled in the box, altering a UL listed product. Again, not code legal.
3. The ground wire from that cable was lazily wrapped around the existing ground without a proper crimp sleeve. Obviously this cable was an "add on".
4. Having all the neutrals (white wires) tied together is common and OK. The only issue could be if one or more of them have a poor or non-existent connection.
5. I disagree that you can solve this with a non-contact voltage detector. They can be very miss-leading.

Lastly, I would suggest a multi-meter and start tracing circuits. You can do that with the power off (and must), but with a non-contact voltage detector, the power needs to be on. My recommendation, EDIT (Thanks Jason) Test resistances first, being sure the power is off, record the readings per wire/cable pair, then plug in a something like an incandescent light bulb, space heater, anything that uses real power (not LED or LCD bulbs) Then test again for resistance (ohms) between each black and white wire on the same cable. Record and mark them, maybe using a sharpie and masking tape. If some are not near infinite resistance, those cables are going to lights. At any rate... After testing for voltage again, If one of them drops to near zero resistance, you found that circuit. Repeat until you've tested all circuits.

EDIT: Forgot to mention to get access for the continuity checks, you'll need to undo the white wires under the red wire nut, but that shouldn't be a problem to reconnect, since it seems as if all the white wires are already connected. Next, the sloppy ground should also be corrected with a proper crimp sleeve.

Somebody bodged it up, not terribly, but not good.

1. two wires under one screw on the switch, not good and not code legal. The correction for this is to remove the wire going into the cable (top middle), and connect with the other black wires in the wire nut in the lower middle of the pic since it's already pig tailed (a very short wire from the wire nut to the switch).
2. The cable coming in from the lower right is coming thru a hole that was obviously drilled in the box, altering a UL listed product. Again, not code legal.
3. The ground wire from that cable was lazily wrapped around the existing ground without a proper crimp sleeve. Obviously this cable was an "add on".
4. Having all the neutrals (white wires) tied together is common and OK. The only issue could be if one or more of them have a poor or non-existent connection.
5. I disagree that you can solve this with a non-contact voltage detector. They can be very miss-leading.

Lastly, I would suggest a multi-meter and start tracing circuits. You can do that with the power off (and must), but with a non-contact voltage detector, the power needs to be on. My recommendation, BEING ABSOLUTELY SURE THE POWER IS OFF would be to take an old extension cord you don't want anymore, cut it off about 6" from the plug, strip the wires and put a wire nut on them, creating a short. Then test resistance (ohms) between each black and white wire on the same cable. Record and mark them, maybe using a sharpie and masking tape. If some are not near infinite resistance, those cables are going to lights. At any rate... After testing for voltage again, being absolutely dog gone sure the power is off by testing the outlet with your multimeter first , plug in your your shorted out plug and check resistances on all cable/wire pairs. If one of them drops to near zero resistance, you found that circuit. Repeat until you've tested all circuits.

LASTLY, KEEP YOUR INTENTIONALLY SHORTED OUT PLUG AWAY FROM CHILDREN, STUPID ADULTS, PUT IT IN YOUR GUN SAFE OR TAKE IT APART AND THROW IT AWAY! ok, getting emphatic here, it's a useful tool, in the right hands.

Somebody bodged it up, not terribly, but not good.

1. two wires under one screw on the switch, not good and not code legal. The correction for this is to remove the wire going into the cable (top middle), and connect with the other black wires in the wire nut in the lower middle of the pic since it's already pig tailed (a very short wire from the wire nut to the switch).
2. The cable coming in from the lower right is coming thru a hole that was obviously drilled in the box, altering a UL listed product. Again, not code legal.
3. The ground wire from that cable was lazily wrapped around the existing ground without a proper crimp sleeve. Obviously this cable was an "add on".
4. Having all the neutrals (white wires) tied together is common and OK. The only issue could be if one or more of them have a poor or non-existent connection.
5. I disagree that you can solve this with a non-contact voltage detector. They can be very miss-leading.

Lastly, I would suggest a multi-meter and start tracing circuits. You can do that with the power off (and must), but with a non-contact voltage detector, the power needs to be on. My recommendation, BEING ABSOLUTELY SURE THE POWER IS OFF would be to take an old extension cord you don't want anymore, cut it off about 6" from the plug, strip the wires and put a wire nut on them, creating a short. Then test resistance (ohms) between each black and white wire on the same cable. Record and mark them, maybe using a sharpie and masking tape. If some are not near infinite resistance, those cables are going to lights. At any rate... After testing for voltage again, being absolutely dog gone sure the power is off by testing the outlet with your multimeter first , plug in your your shorted out plug and check resistances on all cable/wire pairs. If one of them drops to near zero resistance, you found that circuit. Repeat until you've tested all circuits.

LASTLY, KEEP YOUR INTENTIONALLY SHORTED OUT PLUG AWAY FROM CHILDREN, STUPID ADULTS, PUT IT IN YOUR GUN SAFE OR TAKE IT APART AND THROW IT AWAY! ok, getting emphatic here, it's a useful tool, in the right hands.

EDIT: Forgot to mention to get access for the continuity checks, you'll need to undo the white wires under the red wire nut, but that shouldn't be a problem to reconnect, since it seems as if all the white wires are already connected. Next, the sloppy ground should also be corrected with a proper crimp sleeve.