I agree with KME water is entering the rad via the lockshield and exiting the control valve.
Whilst we are talking about rads. There is one rad at the church (large and double) which refuses to get hot. I took it off and flushed it out both ways, put it back and it heated up half way along then stopped getting hotter now doesnt even get slightly warm...?
I did think of having the heating on and turning every other rad off at the valves to increase the pressure.
Sounds like a possible blockage in either supply pipe, or rad itself (scrap that - you flushed it....)
Other option, and possibly relates to O/P as well; is that some TRVs are bi-directional (can go to flow or return), whilst others MUST be in the flow side to operate. If they`re not, it can cause issues like you describe.......