It’s usually pretty obvious which room in the house is the master bedroom, just by the layout of the house. Even if I’ve never been in somebody’s home before, it’s usually quite easy to tell which bedroom is located where, just by seeing the location of the bedroom door. Not to mention that kids’ rooms often have signs and drawings hanging on their bedroom doors and are almost always located in proximity to the second bathroom. At least that’s the way it is in my part of the country.
I don’t know where you live, but I find it highly suspicious that you couldn’t figure out which was the guest bedroom on the first or second guess, and had to try every single door in the house before finally finding the guest bedroom. Then you take offense that they don’t trust you? I say they were justified in their suspicions, and now you are on the defensive. Just say thank you, and stop being so snoopy.