Thanks for all the feedback
Went outside with my metal detector to have a look for it, but before I started swinging, I saw the plug in the grass :p. Still looks intact, screwed it back it. Just gotta buy some blue threadlock, and see how that goes.
I might have to check the bore too, though the mower still spins over, hopefully the damage (if any) isn't bad enough, and I can still use it.
