I preform walk throughs of vacant properties on a daily basis for my job. Typically these properties are foreclosures that the owner has abandoned the home.
My wife and I carry guns with us in every walk through. You never know who may be in there just waiting to jump you. Several times I have found people inside these secured properties.
For my properties no one is supposed to be in them, so if I sense there is a threat, I will assume the ready position with my hand on my gun.
I think I have it better though as my properties are supposed to be vacant and I hardly ever find vagrants.