My 520i E60 always acted that way.
If I use the car for a trip, the leave it parked some minutes or hours, when I get back to the car the air from vent ducts is ALWAYS very hot. It tends to get normal after a while.

It is normal if you car is in the sun for example, but it happens too at night for example. Seems like the air present in air ducts "overheats". More than the external temperature, seems like the air comes..directly from the engine

In the first 1-2 minutes if in summer is a very very annoiyng fact. Someone experiencing this?

 

AC compressor is likely on the way out. common problem on E60. yes, many of us have/had this problem.

 

the problem is WITHOUT AC. the air at start is almost..boiling!