It sounds like you have hit the basics and found nothing so ……….

[Bruno1949]

inspect the tub/shower area carefully. Remove the trim plates around the faucets and shower head if you can. You have a leak somewhere and the tub/shower is a very likely place for it. The odds are that water is getting behind the enclosure or tile and the wall is wet and moldy. There should be an access hatch for the tub/shower fittings somewhere. Open it and I’ll bet the smell is coming from the wall and floor under the tub.

Clean any old caulk out and inspect every joint. Apply new silicone caulk for bathrooms any place that you suspect might leak.

The smell isn’t in your mind, so it’s going to take some digging to find the cause and the cure. Good luck on the investigation.