I bought my car used (lo miles, great condition) from a rental car agency. What if some low life had smoked pot or used drugs or tranpsorted explosives, etc. in it and left traces and I got stopped by the police? What if they used one of those drug dogs? (and yes, I've had it cleaned and vacuumed, but still,..)
You could be in big trouble and expense because of someone elses behavior or actions. Same thing with your doorknobs and the outside of your house. How would you know if someone planted drugs etc. around the perimeter of your house or garage?
I take a lot of short business trips, meaning I rent cars a lot. I always give it a good look under the seats, in the trunk, and all that. But you can't look everywhere. I hate to think that I could take the hit for some weirdo who rented the car before me. Rental car places aren't always the best at cleaning out their cars. There have been many many times that I've found rental agreements and other paperwork from previous customers in rental cars, so the glove box is something that should always be checked real well.