I would think it definately hatched in the greenhouse and actually it was probably an early clutch last year. In the wild in Bulgaria they take 90 to 120 days to hatch. I would think it would have taken the same amount of time in the greenhouse, unless it was under a lamp, than slightly less.
When hatchlings hatch they go straight for cover, so very easy to over look one that was hatched basicly outside. It probably over wintered in the greenhouse and made it outside this spring. So probably didn't do much growing till after this spring.
Nice find Chris
and yes there is probably more.
Danny