oomis, I wish all hosts were like you! I have been in a situation before where I was given the key to an apartment with no instructions, no note, no anything and had to figure it out for myself. I don’t think I tried literally every single door but it was definitely unclear where I was going.
I just think that for the two seconds that the guest will see the bedroom before realizing it is the wrong one, anything that conspicuous would be easy to clean up ahead of time. Or they can spend that time writing a note with directions instead of cleaning up. Either way, if you don’t trust them not to go into your bedroom, you shouldn’t trust them to stay in your guest room.