Over the years I have found here at least that 99% of the drywallers are on piece work from a parent company.I`d uncover maybe 1 or 2 of the boxes on walls but when you find ceiling boxes covered,SD`s covered 5 gangs covered ...........Really simple cure.Call GC or builders supt.Tell them to call the drywall contractor and have them send someone over to uncover the boxes and you`ll return with a trip charge. Or they could uncover them.I didn`t cover them up so,I`m not cutting them out !!!!!!Stick to your guns that one time and the next time (there`s always a next time) they will have someone there right away.I always told the guys if you see alot covered ,make contact the first thing so we could get someone over there right away.