Maybe from lead paint? That's just a guess and the hives/paint would have to be really old as lead has been banned from paint for a long time.
If you’re not part of the genetic solution of breeding mite-tolerant bees, then you’re part of the problem– Randy Oliver
I think I first read about them finding lead in it in the early 90's in europe. The story has persisted since then. So my guess is the source for the lead was from the paint on hives. I'd like to see an updated US analysis, since we removed lead from our gas, paint, and other products decades ago now.
I'd be leary about getting propolis, or any bee products for that matter, from countries with lax enviornmental laws.