"Who is in the toilet?"
Maybe this is different in some other form of English, but I've never once heard toilet used to mean "water closet" or bathroom. It means a physical toilet, with a toilet bowl. If you said this to me in English i would start laughing uncontrollably.
In all of Britain, being "in the toilet" means "bring in a room that contains the toilet".
I thought this was funny too. I would translate it as "Who is on the toilet?" or "Who is in the bathroom?"
谁在厕所 is also fine, and it can also be translated as “Who is at the bashroom?”. So, include 里 if you want to be more specific on the location.
I would also start laughing (American English speaker, here). But this is correct. This is from some dialect of British English (I'm not sure which.) You hear it from Moaning Myrtle in the Harry Potter and the Chamber of Secrets movie (talking about when she was alive, so not actually fitting inside the actual toilet bowl!). You also hear this usage in the season 4 Doctor Who episode "The Unicorn and the Wasp". This episode takes place in early 1900s England, and the usage of "toilet" instead of "loo" is used to identify someone as low class.
It's not just 'some dialect'. It's pretty standard, and not only in Britain. Much less laughable, in my opinion, than "restroom". Seriously, what are Americans doing in there, sleeping?