What does it mean when a 3 year old little girl steals?

my daughter's youngest daughter stuck some cookies in her coat jacket today at target, and we did not discover it until we got in the car. of course, her mother marched her into the store, made her apologize and give the package of cookies back. she then disciplined her and reprimanded her sharply. this is not the first time the child has taken something that is not hers. she previously took something from the mother's day out and was punished for that, as well. is this something she is ingrained to do? we are concerned that she is very young to be already so sneaky.