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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!
Yes, that’s similar to my front door keypad, but more appropriate for in-house applications! A little more low profile than the main entry type.
No inside lock on my bathroom door, but I do have one for the guest bathroom.
Another thing to fix - it might have actually been worse for you if it had been you on the loo and the guest walked in 
I do allow self check-in when I’m at work, and have been working full time at my shop of late, but normally am able to greet the majority of guests during the year. Even when doing the walk through with them, it doesn’t always register, to be honest. That is why I state over and over - in my listing, in my welcome message, and in my guest guide in their room. But if I’ve learned anything in 3.5 years hosting in home, it’s that guests often don’t read what you’ve spent time outlining and trying to make clear. And for a small percentage of them, they do read and understand, but will STILL do whatever the hell they want because they are paying for it and they feel entitled.
Sharing our home with guests has been wonderful. We’ve had hundreds of guests. We end up with a lot of new friends from all over the world. We love it!
I also have created numerous life long friends because of in home hosting. It’s not all bad, I agree! I’ve had guests invite me to their homes abroad, and have been asked to house-sit by multiple international guests. I wish all guests were like this, but I guess we take the good with the bad.
Your post is too funny… But I’m a big fan of “rants”.
We have several listings and we hosts very large groups, so we have to have a LOT of linens and towels and supplies, so need to use several of our closets as storage, so we just put DEADBOLT locks on the doors. (When we just put door handles with locks, some guests would pick them open with credit cards, [which would chip the paint and we’d have to re-sand and paint them.])
So be sure to get a deadbolt that has to be opened with a key or keypad code.
Good luck!
This happened to me too, as I was laying in bed reading. Talk about uncomfortable! I put a keyed lock on my bedroom door, but come to think of it a digital lock would be much easier!
Any guest that did that to me would get a review of all “1” and a comment that they broke into locked areas of the house, recommend not hosting them.
Seriously? They would break in to locked closets? Wow.
I was going to recommend this lock:
because it replaces the typical spring latch lock that most interior doors have.
Yep @CeeBee It used to happen all the time. The worse part was he had all the doors painted a charcoal color to cut down on labor cleaning costs, so when they would to break in with their credit cards, they would chip the paint and the “fresh wood” was very noticeable against the dark charcoal-colored paint… After we installed the deadbolts, the attempted break-in mostly stopped. Now it’s just a couple of times a year.