having fresh droppings at all entrances doesnt always mean the warrens are heavily occupied. depending on what equipment you have, you could always make a few visits at night & quickly scan the area with a lamp or nv unit.
not that i make a habit of it each time but when i want to monitor a set after ferreting it i back fill every hole & to keep regular checks to see just how active the set is, then when i return i send the ferrets down the open holes, then when finished i back fill again..
you say it took 6 hours to do a few sets, did you place nets at every hole ?