156 – What is this warning?

Moin, had seen this warning yesterday and also this morning shortly after starting. But after a short time the light went out again. In the operating manual the signal is there, where with others (probably not Selespeed Alfas) the sign for the trailer coupling is. Looks a bit like an open trunk flap…? ___________ (———-\—-/ (———\/_______ (——————-) (________________________) Who can tell me what this is and what you should do? Thank you 156e r PS: Unfortunately it always breaks the picture, dashed lines please think away ;o)