Some homeowners, and even some techs, will open the drain valve on a ceiling hung expansion tank and drain off some water there. What some people don't know is that these tanks will drain a little water, seem to be empty, and are actually full of water because they get airbound.
To empty such a tank when the water is cool or cold, you need to remove the valve, and stick a piece of 1/4 tubing in through the valve outlet and up to the top of the tank. Have a bucket ready because it will pour out of there.
I know of ceiling tanks that are 50 years old or more. Some get water logged, some never do.
Unrelated to this topic, but if your pressure ever starts to build slowly and blow off the PRV, it's because you have unnecessary water entering the boiler, many times from a pinhole leak in the domestic water coil.