I agree it's not only a leak that would cause the radiator to continually need bleeding - but it can be the cause. So in my opinion it's still worth checking for a leak.
Even though there's no signs of a leak - doesn't mean there isn't one. I've been to jobs where the pressure on a combi has dropped every few weeks and every few months - neither showed any signs on the ceiling, but both leaks were under the floorboards upstairs - and didn't mark the ceiling below.
Just another thought. Is the pump on the flow or the return? If on the return then its sucking the water round and the water in the pipework will be at negative pressure compared with the outside. If you have microleaks in your joints air will be drawn in due to the pressure differential. This has to go somewhere. Not that comon but its a possibility.
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