I've had this happen just recently. I couldn't catch the mouse. I ended up putting a cracker box (about a foot deep and 5 inches squared into a corner. I put bird foot in it and then laid it down in the corner. I then let it set for some time. I waited until I heard the mouse in the box, then went over and flipped the box up, kinda trapping the mouse in the box. The mouse was inside the box eating away. It's the simpliest way I know how to catch a loose mouse.

Good Luck!
Jim Smith