I found this, which may be helpful to you.....or not:

An employer may refuse to pay final wages only if the employee owes the employer money due to gross negligence, willful misconduct or dishonesty. These often are difficult and expensive to prove.

You fall, unfortunately, in this category with not returning the key "in a timely manner" to the store. This could be any of the three. You have no proof of trying to get in contact with the store as you never actually went up there for a few days. Later, QD.