It is a situation where a cold cave can be accessed through a wood trappe.
It was here an heavy foot of a coat-bearer up this who's insulating a part of the trap from the heat of the upper side.
Once removed, it stays a clear mark for some quarters, the little circle is just the metallic ring to open the trappe and a local downsizing of the wood's trappe height.
Français: rémanence
English: afterimage