The rads closest to the boiler and at lower points in the system should be under positive pressure and a weak point would be recognised as a drip. Any points higher in the system and weak could well be under negative pressure.
OK. That is useful.
Any rad above the ground floor could be a culprit.
The towel rad also has an electric heating element that (I believe) only has a rubber seal on. There’s a T piece where the CH system connects in.
I guess I can go around switching off each rad for a day or two. If the air stops, I know it’s the rad that is off.