Shawn, every time I've planted pickling cukes, I've gotten what my Granddaughter & I call "sleepers", like you describe. I think they're ones that just get hidden real well behind the leaves because they're so close to the same shade of green, that you simply miss seeing them until they're monsters!
(Happens to strawberries, too, even though we don't pick them when they're still green! We always start over at the beginning when we pick & go through it all twice, as there's always some "sleepers" hiding from us.)
I gave up trying to raise cukes for pickling, because they can get so big, so fast, that it seems like they're always ready when I'm not! I still
plant them though, & use them just for eating like Weezie does, & for putting in salads.
I'm curious though, about if the monster you found had any other cukes on the same
plant.
If a stray
seed found its way into your Alibi package, every cuke on that particular
plant should be like that. (Otherwise, it's just a sleeper... & they DO get big, seemingly over night, if you dont catch them when they're small enough to fit in the pickle jar!)