"Air" building up as fast as you describe on the towel rail is not ok on a sealed system, the only two normal explanations for this is a micro leak on the system and the pump draws air in through this micro leak and the towel rail is simply the most convenient place for it to gather. Or you have a certain amount of corrosion in the system resulting in a hydrogen build up in the towel rail, which could also explain the "dirty " leak from the boiler, if hydrogen build up is a possibility then there are methods for proving if its air or hydrogen.
Either way, apart from a clean dribble of water after removing the filling loo key, there should be no wAter appearing below the boiler or on the bottom Tray.
Theres a 3rd option ScottishGasMan.
Pinhole plate Hx, common with WB, and would cause all the problems above.