I also have a parts drawer with an assortment of old knobs and I recently had the same experience you have. Out of an assortment of about 25 there were 6 that had an off white coating similar to mold. The coated knobs were identical and appeared to come off the same piece of old test equipment. The rest of the knobs were perfectly normal. Thinking they were moldy I soaked them overnight in a clorox solution and it didn't phase them. So it definitely wasn't mold. But with soapy water and a old toothbrush they cleaned easily so I determined it was an old plastic compound reaction.
Contrary to what some are saying I don't think you have a health problem in your dry basement.
Some Craftsman tools with plastic handles left in a car trunk or hot toolbox for a long time often get real stinky. That again is the type plastic they use and giving them a soapy bath usually helps a lot.