Bladenoobie1, Labor Day is a Federal holiday, no mail delivery.
I have heard of packages marked delivered but brought back to the post office, to be sent out for redelivery the next business day. The letter carrier may be looking to establish that he was there to deliver it but wasn't sure where to leave it. That's what the little yellow slip is for, to let the customer know where his mail is.
If the supervisor is having trouble with some carriers not getting the job right, a service talk in the morning is a good way to update them on proper procedure. Better yet, follow that up by going out in the street after the carriers begin delivery, and talk to the problem people privately.
After all, with the carriers in the street, there isn't much work for all the supervisors to sit around indoors all day. No matter how good the coffee is in the supervisors office.