Hee, one can never know, but the thought came to me that possibly she is indeed "marking" you in ownership, at least, of the hand that feeds her. Check this out: Other animals do it as territoriality, right? Well. She may have deduced that The Hand is NOT s'posed to be EMPTY; thus some OTHER animal may have gotten to what she considers her stash, a regular place where she gets her food.
Heek. It is possible, you know. So she may have marked you to cover her bases: If another squirrel stole from The Hand what she believes is for her, she wants the other squirrel to realize: This is HER The Hand.
I know. Hee, maybe not. But it is possible! I am discovering that squirrels tend to have a reason that makes sense for every thing that they do.