You won't notice it this coming winter either, I'd guess. I know JUST EXACTLY the smell you are describing. You can duplicate it by turning on a very dusty steam iron. LOL. I get it sometimes too on hot water heat radiators or around our heating system in summer. Dank, sweet, dusty smell. Humidity, dust, confined air flow. You can get those long, pliable feather or some mircle fibre dusters to back into the labyrinth, or set a dehumidifier or fan near the units for a couple of days to dry out the wall area behind it. We turn our whole system off in summertime (no sense keeping the boiler cranking), but if you have the ability to kick it on without too much trouble for an hour or two in the evening, you may want to do it. Take the opportunity to bleed out the air from the lines at the risers. I have had a riser stuck open (we have the older ones fashioned very like a bicycle tire valve). It seeped water just enough to make the area damp by the radiator, and of course we got 'that smell'. Good luck.

Thank you for validating my sense of smell! Your explanation makes a lot of sense, especially since we've had an incredibly wet and humid summer. I'm sure there's at least some condensation back there. I have a fan trained on the radiator right now and will see if I can bleed out the air.