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Guess what I do if I see a sign saying ‘Keep Off the Grass’. 
And ‘Wet Paint’ signs are just begging for me to touch the surface in question.
But yes, I do tend to have an overly optimistic view of human nature (even after all these years of hosting - weird really!) but nevertheless I can’t understand why someone would use a bathroom not intended for their use when they’ve got a perfectly good (unoccupied) one expressly for their own use.
LOL. Just “let the mystery be” I guess.
Put a digital lock on it, for which you have the code. I’d do the same for your bedroom, too.
We did that with our bedroom (our bath is en suite). No one can accidentally walk into our room. It has a sign on the door saying it’s our room, but two guests walked in before we got the digital lock. We have the same kind of locks on our guest room doors and our front door.
OK, you can tell I have some free time on my hands today!
I will stop now . . .
I’m surprised nobody has ever written a detailed how-to list for shared-space hosts on guest-proofing your home. Lock up your valuables and lock up your personal space should be #1 and #2 on the list.
My hat is off to the shared home crowd, I could not do it and as a guest I would not book a shared space. I guess that makes me “new school” Airbnb!
But IF I did, locks locks everywhere!
RR
I hope she was startled too. What did she say to you?
I host in my home too but have only once had a similar issue. An Airbnb employee/guest used the shower room belonging to another bedroom/listing. Her excuse was that she needed a pee and one of her mates was in their shared bathroom. They were mortified; she was simply entitled. What enraged me most was that she had to have snooped around the house to find it.
I am assuming that you allow your guests to self check-in if you are working and not home until 6.30pm? I think that might be the problem. By checking people in personally, I can make sure that they really do understand what is their space and what is not available to them. Or perhaps you could have a sign simply saying “Private”, leaving off the fact that it’s a bathroom.
Can you put a lock on it? When there are no guest, of course, it would be unlocked. If you have guests, then lock it with a key. It will be a pain in the neck each time you need to unlock it for personal use but at least it will keep them out. You can put a lock box on the bathroom’s door knob so you can retrieve the keys when needed.
She needs something like this:
Haha, I love this answer!!
I would lock the door when you are not using it. A pain in the ass, true, but I think it is the only solution.
We used to live in a condo, with a bath on the first floor, but my MIL would pass by it, go upstairs, and use our ensuite bath. Drove us CRAZY and we never did understand why … People are goofy, ha! Lock 'em out. 
I’d really like to know this too. But also, what did the OP say to the guest?
And I’m so looking forward to reading the review.
KKC, you’re right! That’s perfect.
Rude, rude, rude. And one doozy of a review.
I wonder why the peeing guest didn’t lock the door?
Perhaps to get caught? An exhibitionist perchance…
Same here. My master bedroom has an ensuite bathroom and my space is totally separate from where guests are. They have a full, private bathroom as well as access to a lovely outdoor shower with all amenities. Yet, imagine my surprise when I came home to a guest in my shower in my early days of hosting. I have a coded lock on my bedroom door now. Solved the problem. I also have one on my indoor storage area as well as the front door and garage door.
It’s becoming increasingly difficult, honestly. I have some stories, for sure! Also, I travel almost exclusively with Airbnb and have only once done a shared space, and didn’t care for it - I only search private spaces now. I can see the draw especially for foreign guests, though . . . to really experience how someone in a different area lives and get an inside look into their lives.
@Joan @jaquo She seemed startled as well, and as I replay it, it’s almost humorous. I remember saying “Ohhhh!”, and having my hand on the doorknob . . . telling myself to “SHUT THE DOOR!” in my head . . but stood there probably too long until it all registered and I actually shut it. I think I said “this is my bathroom” and she may have said she didn’t realize, or I’m sorry, but I don’t recall exactly. I followed her comment with, “there’s a sign on the door”.
I’d like to add that I had the former guest back in Jan '19 that I’d hosted 2x that criminally trespassed in my home eight days after his last check out, while I was asleep, and found him in my kitchen after my dog alerted me by barking. I think I have a little PTSD thing going on due to that, so this woman in my bathroom startled me more than is should have, and nearly sent me into a panic attack. I just had to pee, and didn’t expect to walk in on a woman with her pants around her ankles!