I've never even considered snooping in another person's medicine cabinet. If there was no toilet paper on the holder, I have opened likely cupboard doors, but didn't pay attention to anything else that may have been in the cupboards.
After being told by a visitor that he has helped himself to small quantities of medications containing codeine, etc. from other people's medicine cabinets, I moved my prescription medications to my ensuite bathroom which is not used by visitors.
My embarrassing moment wasn't really snooping, it was curiosity. Staying with in-laws in Hungary, I came across set of very small plastic drawers on their kitchen counter. Each drawer was labeled with the name of the spice or ingredient it contained. Most names I recognized, but while I could tell that "porcukor" must be some kind of sugar (cukor), I wondered what "por" meant? When I pulled open the drawer to look I discovered that the drawer was not nearly as deep as I expected. It pulled completely out from the holder, tipped and spilled a cloud of powdered (icing) sugar over the counter.