On the assumption that there isn't a fundamental design issue with your system I would assume the more obvious reasons and rule them out first. When you are next there I would try the following

1. Check the header tank to ensure it has water in it and the ballcock hasn't stuck and is working correctly (likely to be the cause I am guessing )
2. If that is ok. With the hot water and heating off, bleed all the radiators. Check that the header is filling correctly to ensure it never goes below the top of the feed pipe during this process.
3. If this doesn't fix it and / or as an extra test, put something over the vent pipe (ie the one that dumps any expanded water into the header tank) so you can tell whether water is being pumped out of it or air sucked in. Could use a cloth for the former. For the latter you could seal a bit of balloon or similar over the end and you would see it getting sucked in or suspend a tub of water such that the end is submerged. If it is sucking water will be sucked out of the tub.

If none of this works post the results here and we could go to phase 2