ZUMA has been wrapped up in the mystery surrounding USA 276 and the ISS since last November, when Marco Langbroek found that its launch window and trajectory lined up very closely to their orbits.
After a few delays (with little insight into their causes, much like the launch of USA 276…), things seem to be lining up again:
The new launch window for January 6th is the same as it was in November: 1:00 UT to 3:30 UT. This excludes a launch (exactly) into either the ISS or USA 276 orbital planes, as the latter only pass over the Florida launch site after the launch window has ended.
This would seem to suggest that the coincidence in time of the launch window and orbital plane passages in November was indeed coincidence (but there is a "but": see below...).
On January 6th, the orbital plane of USA 276 passes over the launch site around 4:27 UT, an hour after the end of the launch window. The orbital plane of the ISS passes over the launch site around 7:04 UT, some 3.5 hours after the end of the launch window.
The launch already has slipped one day, and a few days further delay would slip the passage of the USA 276 orbital plane back into the launch window, as the moment of orbital plane passage shifts about 24 minutes earlier in time each day. This would happen already with a 3-day delay in launch. And a further delay eventually would do the same for the ISS orbital plane passage after several more days.
And the launch has already slipped yet another day since this post.