2010 Legacy GT ltd-Part number that controls dome lights going off

[garygbnj]

Problem: Once every 2 or 3 months the battery is dead. Car is garaged at my home in FL. Mechanic cannot track down the source. A couple of weeks ago my wife went into the garage about 15 minutes after I got home. The dome lights were on and the door was shut. I opened the drivers door and shut it and the lights went off in a minute or two. My mechanic cannot find the part number for what controls the amount of time that the dome lights stay on. Does anyone know the part number for what controls the time the dome lights stay on? Thanks. Gary