What happens to a FedEx or UPS envelope that acidentally gets put into a blue US Postal Service box?

Sometimes when the postal worker gets the letters out of the box, if there is a FedEx or UPS box right next to it, they'll will place it in the correct box (FedEx and UPS will do the same when they find mail in their boxes). Most often however, the mailman will return to the post office with the competitors letter. The Post Office will then call Fedex or UPS and let them know they have one of their letters and ask them to come pick it up. Be warned though, I've seen the postal service hang on to one of these letters for a week before they call.What happens to a FedEx or UPS envelope that acidentally gets put into a blue US Postal Service box?
fedex employee gets called out to postoffice,

or they return it to sender