There is a smell in an interior bathroom with no windows. The tub is rarely used. When I noticed the smell, I got on my hands and knees to sniff everywhere, stuck my head in the toilet tank, shoved my nose in the tub drain, put my head in the vanity, and so on. Nothing smells up close, so I threw bleach down the sink, toilet and tub. Since the tub hasn't been run since I last cleaned it about 3-4 months ago, I figured that was the culprit. I ran hot water in the tub, cleaned it again for good measure, and thought the smell went away. That was when I noticed that the smell goes away when the AC is running. If there is no air being pumped in, the smell is there. That allowed me to rule out a dead critter in the duct work. Any ideas???? Thank you!!!
To me it sounds like the vent from the bathroom to the outside must have moisture in it and growing something. I say this because many HVAC systems have a small percentage of fresh air added when they are running and this keeps the air moving out of the bathroom. When the System is off the smell wafts back into the room. You may be able to use a spray disinfectant, Remove the cover, turn the fan on and direct the spray into the fan (not the motor) and spray for a few seconds,,, or minutes. put the cover back on and see if the smell is still there in a day or so. I am guessing you have a fan since there are no windows and this has been code for decades.
Just a thought, I worked in an office with an internal bathroom, i.e. no exterior walls and we noticed a off smell there as well. The toilets and sinks worked fine, it wasn't until we realized the floor drain was the source. There was a P trap on the floor drain and the water had evaporated in the trap, allowing air to escape into the room.
You might want to make sure there is enough water flowing in the tub drain to seal.
The other possibility is there is a negative air pressure and air is being pulled (bubbling up) past the gooseneck or P trap into the bathroom. that might be why the smell goes away when air is being pushed into the room by the AC.
Looking under the vanity, does your sink drain have a simple p-trap that goes straight into the wall? Or is there something more in there, something sticking straight up with a bunch of vent slots/holes in it? You could have a failed air admittance valve, allowing sewer gasses to leak out.