[NautiNurse]

The ones that you don’t see coming end up hurting you.

Didn't-see-it-coming would fall under the category "unforeseen."

If nobody saw him hit his head, that would be "unwitnessed."

"Unseen" is a certifiably useless description.

[Leep]

If noone saw you get conked on the head..did you really get conked in the head?
Sort of like the tree falling when no one is around.