Gap under door.

From a mouses's perspective, an open door is far different than how we view an open door. For a mouse, a "closed" door can still be open as long as there is some gap - and usually that gap is at the bottom of the door. A common way for vermin to gain entry is damaged, worn, or missing door sweeps. Here, the front door sweep was cut too short and did not extend the entire bottom of the drawer and the back door sweep was not properly installed. it was crooked leaving a gap that was easily big enough for a mouse to enter.