Is there a mouse in your house?

And if so, how do you know?  A few weeks ago we left the garage door open all night.  I thought, I must have a mouse now.  So, I put out a couple of traps, baited with the old standby peanut butter, and after 2 weeks, nothing.  So I told my wife, "we lucked out, no mice".  We then went on a little vacation to the New Jersey shore for a few days.  We returned yesterday, and what do you know?  One little guy bought the farm.  So, I'm wondering, if you only have one or two little guys and they are new, how would you know they are your new guests?  I put a new trap out just in case.  And yes, with grade "A" peanut butter.  There's  nothing like going out in style.