Why would my son's bedroom wall be very warm to the touch with no outlet's even near?

If it's an exterior wall it could use more insulation in it to block the sun's heat or in winter the cold. Or if it's near a kitchen or bathroom it could be the heat from the hot water pipe. Also wall receptacles do not get hot unless over loaded. How long has it been hot? I noticed it 2 days ago. It's in my son's bedroom, right next to where he sleeps, of all places. So I'm concerned. It's about a 2' x 2' area and it is an exterior wall. There are no outlet's on that wall at all.