Dar bar is acting very strangely.
Lately, I have heard multiple 163 rocket engines near me at 40,000 feet before any dar bar shows up or I find bombers 25k over our strats without a dar bar having been generated anywhere along the bomber's course.
I haven't had a chance to experiment, but there are two settings that look like they control darbar: SectorCounterRange and SectorCounterAlt.
The default setting for SectorCounterRange is 63360' (12 miles) - which match the radar circle, fighter and bomber warning ranges.
The default setting for SectorCounterAlt is 250' - so no darbar until someone is above 250'.
So it would seem as long as nobody is near enough to a radar source (I'm guessing any base/strat/etc.) or above 250', there will be no darbar.
Although in EW/MW I've personally seen no darbar until the con showed on dotdar once they broke into my base's ring. It would be interesting to find out if the SectorCounterRange also includes proximity to enemy players, so a darbar could pop if they're close enough to a plane/vehicle on their enemy's side.