Central heating losing water

[Jimmy3]

I have a sealed (combi boiler) central heating system and have noticed one of the top floor radiators slowly fills up with air over a 3- 4 week period, meaning that i have to bleed it and then top up the water at the (Vallaint) boiler.
We have a town house, so three levels. The boiler is on the ground floor and there are radiators (9 in total) on all floors. The rad in question is on the top floor, however the other two rads on that level, as well as all the other radiators in the system, don't seem to be affected.
I have checked the obvious like the boiler high pressure relase valve and there are no obvious signs of water leaks in the boiler itself.
Can anyone suggest any tips to finding where the air is entering the system without ripping my house/floor to bits in the process?

 

[swbjackson]

It may not be air, it may be hydrogen gas caused by corrosion. The next time you bleed the rad hold a cup upside down to catch the gas/air and poke a lighted match up inside it. If it goes pop it's hydrogen and a by product of corrosion.

If it is just air then you must be losing water for air to get in.

 

[Jimmy3]

I checked the gas and it is not flammable. Also the radiator in question only six month old.
I guess i must have a small leak somewhere.
I have tied a bag around the pressure relief valve outlet which doesn't show any sign of collecting water.
Does the fact that the air only seems to collect in the one radiator mean that the leak is local to this or is because it is the furthers/highest radiator from the boiler?
There are two other radiators on the top floor which don't seem to be affected.