As I remember, people with fairly recent model BMWs have mentioned this problem before. The only way it makes sense is that the hot air ducts for the vents are spewing hot air into the space behind the dash or that the ducts run very close to the empeg and are radiating heat to the empeg.

Spewing that hot air into the dash area sounds like a bad idea. Maybe there's a leak in the ducting somewhere? Can you check for that? If it's just because it's so close to a duct, maybe you could insulate the duct?