Please do not "borrow" empty moorings. Once you have done this on a regular basis, you will understand why this is inappropriate mooring etiquette.
We have always kept our boats in a marina slip, but were recently granted a mooring certificate after being on the waiting list for 6 years. The mooring is located in a very sheltered cove within a local island nature preserve. This cove is beautiful and has become very popular with overnighters and daytrippers, filling up quickly every summer weekend. Our purpose is to use it as a local retreat, since it's not far from our marina.
Some boaters don't realize that these are all private moorings, which cost the owners good money to install and maintain. They also think that since it may be empty late in the day, it's OK to pick one up and spend the night. Nothing is more annoying than for us to leave work on a weekday or Friday afternoon and sail to our island retreat, only to find another boat on it.
It is also an embarrassment to the "borrowers" to be asked to leave - especially while grilling their dinner over cocktails. We once had to wake people, arriving late after returning from a long cruise. Since no other moorings were available, they had to drop
anchor in darkness.