About 6 years ago, we had a period of having wasps in the bathroom. I found out they must have been coming in through the fan (which had lost it's cover outside, due to the absolute cowboys who did our bathroom, putting it on with silicone sealant and not screwing it in). DH put the cover on properly.
We still found the odd wasp, mainly due to the fact that the slats on the cover, kept staying up - DH sellotaped them down (we don't use the fan, it is huge and our bathroom is small with two windows).
A couple of days ago, DH found a giant wasp in there. Although the fan is taped up outside and the windows were closed. He said it must have come in through the trickle vent. I closed the trickle vents.
This morning I was on the loo, when a buzzing caused me to run out of the room. DH at work, so I grabbed a can of Raid, and sprayed the giant (it is humongous), whilst it walked around our toothbrushes (which I guess are going to have to be binned now).
After a while I went to check in there (with DM on the phone for support), the giant was laying on the floor but still moving, so I squished it 
The floor was wet from Raid (I sprayed it again to be doubly sure).
The windows are now open in there, but the room is heavy with Raid scent. Poor DD needs the loo, as do I , but I'm too scared to go in there.
I have no idea how it got in. I've checked outside, I can't see anywhere. The only thing I can think of, is the fan cover has gaps around it, which would lead to the large, round hole. The fan inside has no real cover and the outer edges are pretty open; it would let a mouse in, so no problem for a wasp...if that is where they are coming from.
I should add that I am extremely phobic about wasps. Enough that I have run in the road and tried to jump out of moving cars if they are around. The first time we had them in the bathroom, I was having EMDR and the therapist tried to use that to stop the phobia; it didn't work.
Where are they coming from?