she was probably either a lineolated parakeet or a grass keet which actually come from different parts of the world so they may very well have different breeding needs, also, I doubt the pet store is trying to breed her so the probably planned on just taking the egg away at the end of the day and hopefully replacing it with something
go back in a few days and if shes a) still sitting and EGG not a marble or anything or
b) has laid other eggs that havent been taken care of
I would say call animal control...
but im a bit of an extremist sometimes.