There are different customs for use of empty moorings depending on where you are. Some are enshrined in law or regulation like the South Cove on the Connecticut River which requires owners to designate moorings that can be picked up by anyone if the owner is vacant. It think that is part of the agreement with the state or feds that dredged that mooring field.

Others like Hamburg Cove further up on the Connecticut River have rental moorings, but the prevailing custom is that if an owner mooring is vacant you can use it but be prepared to leave if the owner comes back. This custom helps to make use of a scarce resource because the great majority of these moorings are not used as the normal mooring place for the owners boat. They are mostly vacant on weekdays.

I would never tie on to a mooring in Maine in a lobster boat basin as the odds of the owner coming back are strong. But there are lots of moorings in remote coves, many of which were put in with no permit right in the middle of the best anchoring location that I have no problem using. I will move if the real owner returns.

Active Captain explains the rules for some like those above on the Connecticut River, but there is no way for the transient boater to know what the rules are for most moorings.

So, I don't know what kind of mooring the OP has, but I wouldn't be too bent out of shape if it were mine and someone grabbed it.

David