This page has a thing that looks like yours, #385:

http://pestcontrolcanada.com/Questions/pest_photos_301_t.htm

"This appears to be a puparium of a higher muscoid fly (Diptera; suborder Cyclorrhapha), a very large group that includes the common house fly ... When the maggots are mature, they usually wriggle away from the food source in search of a drier place in which to pupate,"

So it would seem the maggots have moved AWAY from somewhere and not necessarily anywhere near where you saw most of them, that might have just been the most appealing place. Any old food left around there?