Has she got a thirsty dog? :lol: only joking. Only explanation would be the pan has a hairline crack below the waterline and is seeping away directly under the pan..............
If all the other rads work ok try turning them all off except the one that is not working and make sure the valves on the one in question are turned on.
has it got a themostatic rad valve?
when did it stop working?
polythene, paper, cardboard, bolt, nut, washer or screw just about any thing can get under there depending on the casting you can get a mirror and check under the rim for anything obvious.........
You mentioned water seems to fall more on one side of the pan than the other could be a badly cast pan or bleach block / blue loo etc washed down under the rim of the pan which happens with with these new flush valves................ :)
Umm did the gas man not mention puting a cleaner in the system? And then a corrosion inhibiter in the system, otherwise he will be back before you know it! :) i