I posted here a little while back about a drain fly problem in my basement, however I am concurrently also having an issue with small fruit fly-sized flies in my bathroom. I don't know what exactly they are but when you smash them they leave a powdery residue much like drain flies do.

Biggest thing here is I can't seem to identify the source. When I first noticed them I did the baking soda-salt-vinegar solution into the sink and shower drains but this did not seem to reduce their numbers at all. I noticed some that looked to be coming out of the window sill, I cleaned it out and sprayed it with flying insect killer but that hasn't seemed to have helped. They honestly seem to be completely immune to regular flying insect killer unless I spray a ton of it.

Is there any good way to trap these things reliably and how can I figure out where they are coming from? I'm getting tired of killing 20+ of them a day...ugh. I put out a regular sticky trap with colors that is supposed to attract fruit flies and gnats, it has caught a few but not nearly enough, they are still all over the bathroom.

 

Proper ID is necessary for fly control as their harborage and food sources vary widely among fly species. I suspect it is related to moisture issues though based on previous posts. If you can post good, clear pics of them on the glue traps or otherwise, that could be helpful for ID.

What is beneath the bathroom, is it basement? If so, inspect the wood and pipes for moisture issues. Another idea would be to put clear tape around the base of the toilet to see if they are coming through the gap between toilet and floor.

Is there access behind the tub faucets that can be removed for inspection?
There are no baits for the smaller fly species. Even if there were, without resolving the harborage issues, it wouldn't solve the long term problem.

 

I'll try to get a pic but it may be hard to get a clear one.

Under this bathroom is just crawl space. I'll have to try the tape trick.

There is access but it is kinda blocked by a cabinet currently. We just had this bathroom remodeled last year, the flies showed up about 5 months after. They also appeared shortly we started having an issue with the drains acting like the vent was blocked (gurgling from shower/sink drain when you flushed the toilet) and I think they are connected.

 

I'm thinking that you're correct about the possible plumbing issues being related to the flies. Pics probably aren't that necessary in view of the known information. Surely a type of drain fly. It's possible that the P traps aren't staying full of water and the flies are coming through the drains. Even cleaning them won't help if they don't stay full.
Close the drains and see if the number of flies lessens. I've heard of people inverting drinking glasses over top of the drains after lightly coating the interior with cooking oil so as to be sticky enough to catch flies. Never tried that, but it sounds feasible.

Can you get into the crawl space to inspect the underside of the bathroom? I think that is important.

 

So I did manage to get a pic of one that I sprayed last night. If these are drain flies they are smaller than the ones I am used to seeing.


I'll have to try your idea. I can't close the shower drain but I could try the glass trick. Also unfortunately the crawl space under this bathroom is not accessible to my knowledge.

 

Good pic. It is what is typically called a drain fly or sewer fly. Search "psychoda".

Being in pest control for as long as I was, not being able to access/inspect a crawl space nor have any idea of moisture levels, would make me nervous.