OK, looked up propolis in my ABC XYZ.
Page 671 Paragraph 1
"Honey bees have a threshold of acceptance that is variable, that is, when they cannot find a material they seek such as pollen, nectar or propolis, they may accept a substitute. Bees have been known to collect and use road tar, drying or soft paint, caulking compound and similar items in place of plant gums and resins. This contamination with modern products makes propolis use in any modern medicines questionable. Bees have also been seen gathering propolis fro abandoned bee hives."
So is there any way to tell or refine it. Or is it small enough to be insignificant, I can't seem to find much reference to it. I swear this stuff smells like 80% caulk. Maybe some of the resins are similar?