Hi, starsfan, and welcome to FlightAware.
I haven’t been following their schedule lately, but they’ve been experimenting for several years with a HOU-EWR-HOU route. In fact, our office has one of those “Work Hard, Fly Right” ~9’x5’ banners advertising this route that reads, “THE ONLY DAILY NON-STOP TO NEWARK/NEW YORK” that was originally posted inside a HOU passenger terminal. That said, I don’t see that flight on FlightAware now, so it may not be operating anymore. It was marketed as a convenient way to get from central Houston to New York in the middle of the day. However, they operated it with 737-500s which I consider to be the ugly duckling of the fleet now that they’ve gotten rid of their MD80s. I always chose to fly 777-200ERs out of IAH when going to Newark/New York, even if the drive to IAH was 10 minutes longer.
I think you’re right that the HOU-SAT-HOU flights are not in the schedule; they’re repositioning/maintenance/test flights. I believe that Continental does some kind of maintenance at HOU, which is why the planes are there in the first place. I’ve been seeing COA flights in and out of HOU on FlightAware for some time and can definitely attest to being given a “no delay or hold short please” takeoff clearance at HOU and then hearing, “continental niner niner niner zero, traffic departing prior to your arrival . . .” around midnight.
I believe they reposition aircraft for maintenance around that time as COA999x IAH-HOU and then reposition them afterwards as a similar call sign or COA19xx.