Sounds like a slow leak. On a number of occasions I've seen it coming from the pipework underneath the boiler, and on a decent number of those it's been on a compression fitting (just needs tightening).
Also, it may be worth running your hands underneath both valves on all radiators. Again, it could be a simple case of tightening a nut if you have a slow leak... if this is say, in a bathroom on a tiled / lino floor, it can easily missed / dismissed as water from your feet when stepping out of the bath / shower.
If you're unsure, though, always get someone in to scope it out for you.