Exhaust mainfold leaking

[The motor guy]

Okay, I know this isn't a boat forum, but I got my manifold off on the other side on my gm350 marine engine. Somehow the manifold was leaking, but not quite enough to get into the cylinder. No rust on the spark plug, exhaust valve or the cylinder wall on that port. I used my borescope to take a peak inside

 

[The motor guy]

Must have been slow enough and small enough of a leak that it evaporated before getting into the cylinder. That's my theory anyways. Opinions? I went too long on these manifolds, lost track of time. Opinions?