Wall is warm right by vent in my bathroom?

[CJ]

The bathroom in my apartment has one vent in the ceiling and one on a wall by the door (same wall as closet) near the ceiling. Near the floor to half way up the wall is warm, but not in my closet. Is this something I should worry about? Or could it be someones heater pipe? I am on the 4th of 5 floors and we have those old cast iron radiators in all the building.

Thanks!