Haven't seen this before - noticed this worker on an empty box while I was doing an inspection, picking off old (soft today in the sun) propolis and packing it onto her hind legs -
(http://s21.postimg.org/dqtb76xub/Gathering_propolis.jpg) (http://postimg.org/image/dqtb76xub/)
I have seen them pulling off old propolis on my frames I have removed from the hive.
Here are some doing the same:
http://bushfarms.com/beesmisc.htm#propolis (http://bushfarms.com/beesmisc.htm#propolis)
(click on small pictures for the larger ones)
This one is gathering the rosin/beeswax mixture I dip my hives in off of my gloves:
http://bushfarms.com/images/RosinWaxAsPropolis.jpg (http://bushfarms.com/images/RosinWaxAsPropolis.jpg)