We have had mice in the house off-and-on. I agree that if you find one, there are likely to be more. We use bait in live traps, and hubby takes the traps outside far away from the house and releases them into woodsy areas. Fall is the time when they are most likely to sneak into the house for warmth.
Also, what we found is that they were squeezing in through the tiniest of openings between the foundation of the house and the rest of the house. Dh went all around outside and pushed wire mesh up against the foundation. This drastically reduced the problem. Now we get only an occasional mouse who is able to slip in through a briefly opened door. Check around the corners of your garage door too; dh closed up an area there where we think they were sneaking in.
It's a little more work for us to bait live traps and carry the caught mice out to the woods, but worth it. Also HUGELY worth it not to have foam insulation bits strewn over my quilting frame! Before we got smart about the wire mesh, there were times when a mouse would appear on the lighting soffits in the living room and watch us watching tv!