Today I found the small bit of plastic below in the engine compartment of my boat (stbd side).
I tried to check if there are other similar ones around, but couldn't find any.
Thanks in advance for any ideas about where could it come from.

PS: the mobile phone is just to give an idea of the size.
Besides, the small part in the bottom corner, almost detached, seems burned/melted.

that is the end of a zip tie that probably came from the rigging under the gunnel

Looks like the white piece of plastic that is used as a isolator on the fuse that connects the positive wire to the positive terminal on the starter. I would look at the starter and check the connector. I have replaced that on my 496HO before. If you take it apart it looks like a fuseable link.