Mouse problems is one thing I know. 
Mice will come in, in search of shelter, no matter how clean the environment is. I guarantee you there is not, and never has been, food under the seats of our motorcycles, on the engine block of our car, or anywhere in our garage. (Except for the seeds the mice brought in, that is.)
Most buildings are not so tightly constructed that mice can't get in. They will swim up drains, and all they need is a 1/4" gap between the foundation and the top of the house; between the soffit and the roof; etc.
Poisoning rodents in a building is a Very Bad Idea, because they will go into the walls to die, and the odor will linger for weeks or longer. Enough of them die on their own in the walls even without poison. 
Here's what we use. It's a very simple and ingenious design: the door only swings one way. Peanut butter works really well as bait.
There are only two difficulties, and they're minor. First, since the trap door relies on gravity, you have to be careful transporting the mice to where they'll be released. And you have to take them at least a mile and a half (as the mouse runs), or they'll come back.
Speed comes from what you put behind you. - Judi Ketteler