The maggots usually seen are the 2nd stage of development for blue bottles or similar flies.
The complete life cycle of these flies starts with the egg laying usually in meat such as discarded beef burger or chicken in the bin or a dead rodent or bird.
The second stage is the maggots or larval stage. A dead rodent will support 4000 maggots which eat away at all the meat off the bone.
We generally find they migrate at night or when its humid and will be drawn toward the building light.
There are limited options for control as they are not an insect at this stage spraying insecticide has no effect. Brushing them up or getting the bins emptied or carcass removed are the main control methods.
A diatomaceous earth dust or new generation of larvae asphyxiation are occassionally used.