This can happen on regular listings, too
I was following a listing that continued to show as "active" when in fact the sellers have accepted an offer, and were just waiting for the buyers to release their contingencies. What the agent should have done was to show it as "pending show for back ups" or even "contingent" (although our rule for contingent is that this is contingent on the buyer selling his own house)
Oftentimes, the onus is on the listing agent to change the status of the MLS.
To determine the action taken, we ask that the listing agent indicate on our offer when it was presented to the seller, and if the seller rejects the offer, to sign that page. But not everyone does this.
So we are relying on good faith and goodwill.
In the case of bank-owned properties -- it may take another 3-7 days after acceptance for the bank's representative to sign/ratify the contract. Upon receipt of that written ratification, the agent should indicate the status of the property.