Whether feral or cultivated, there is some parasitic hive or hives in my area that are like Terminators! They don't stop - sunrise to sunset. I can't even get to my hives to inspect them.
There are two things you could do. The first is to check for sure that the robbers are coming in from outside the apiary. Simply block-up the hive entrances during the late evening, and check those entrances again first thing in the morning. If the robbing should then be shown to be an internal apiary issue, the solutions are fairly easy. BUT - if the robbers are indeed coming-in from outside, that's a b-i-t-c-h.
Inspections ARE just about possible during an outbreak of robbing, if you make yourself a 'Robbing Cage' - this is the type Laidlaw used:
If you make one panel hinged - like a door - then the cage can be placed over the target hive during the night, and the door swung open briefly to let the beekeeper inside next morning. Hardly an ideal way of working, but better than nothing.
I use such a cage to prevent robbing from starting-up when inspecting nucs in late summer.
LJ