I would suggest they are merely high alt cons inbound from another adjacent grid square
Often there is no darbar at all, neither in that nor in any adjacent sectors.
Two weeks ago I ran into what I'd thought to be a Bish NOE strat raider:
As you can see, no darbar anywhere (and yes, our HQ was up, I checked for it)
But then, a few minutes later it turned out to be...
Altitude readout from the film viewer: