This may be helpful.
link:
Pressurization Of Closed Hydronic Systems
Check to see if there are any automatic air purgers (example maid o mist) on the system that may be allowing water out of the system.
Quick question, is there a boiler temperature controller that ramps the temperature up with a drop in outdoor temperature?
The reason I ask this is because I have seen this type of controller take the system temperature/pressure up when the expansion tank is full of water/no air space to allow for expansion. When the pressure goes up, the water is forced out of the system on a relief valve (over pressurization). It operates ok until the outdoor temperature goes up and the boiler supply water temperature drops. When the temperature drops so does the pressure and then you see a short of water situation.
Suggestions:
Make sure that the expansion tank has the proper air charge if it is a bladder tank.
These do go bad on occasion and they lose their air charge.
Make sure that there is an adequate air charge in a non bladder type pressure tank.
Make sure that the expansion tank is large enough to accommodate all range of pressure and temperature for the system size.
Make sure pressure relief valve is not leaking. Often these will weep or outright leak. If it is piped to drain, put a pan or a bucket under it to see if it is releasing water.
When adding water to the system, monitor pressure until the loop is back up to temperature.