It isn't clear whether you have a separate bathtub and shower or a combination (tub with shower head and curtain). I'm presuming the latter but the ideas below apply equally regardless.
Incidentally, the joint between wall tile and the tub (or shower pan) looks beautiful with freshly applied caulk -- but in many installations it should not be caulked. This is because any water that does get through the grout needs a way to drain out of the wall. The tub (shower pan) should have a lip that goes up into the wall, behind the tile, to catch that weeping water and bring it out of the wall. Caulk at this joint seals that water into the wall and encourages mold, rot, etc.
Cutting out the damaged drywall from the ceiling below for a visual inspection was a great start. Unfortunately, "continue cutting inspection holes until you find the source" is the most direct next step. If you're inclined, here are a few tricks you can try first that might help you avoid cutting any new holes:
- Some bathtubs (whirlpool tubs, in particular) have a removable apron panel across the face of the tub. If yours does, remove that apron and find out what can be seen behind it.
- The shower head is attached to some kind of arm (pipe) going through the wall, and it likely has an escutcheon (trim ring) to conceal the hole in the wall. Slide the escutcheon away from the wall and look into the hole to check for signs of water.
- The shower/tub filler valve comes through the tile work in one or more places. Like the shower arm, each of these has an escutcheon/trim to cover the hole in the tile. Disassemble the valve/handle/etc as necessary to remove the escutcheon, then look into the holes to check for signs of water.
- For both #2 and #3, it might be useful to "swab" the piping, valve body, etc inside the wall with paper fixed to the end of a stick or wire. If it comes out wet you'll know the water source is at or above the location where you swabbed.
- Try a borescope camera. These can be obtained through the usual online marketplaces at surprisingly low cost and with USB or wifi connectivity. You may be able to insert a camera through the shower head or valve tile openings, or through an existing or new hole in the floor (near the drain) to get a view before cutting more drywall.
- If the shower valve is in an interior wall, cut an inspection hole in the other side of the wall. Think about repair and restoration when planning where to make the cuts. Make the hole size and location such that you'd have all the access you needed in case you find that the valve needs to be replaced. Make the vertical cuts centered on the wall studs so that the section of drywall can be easily re-installed after inspection/repair is completed.