[Just speculation]

It could very easily have been a plane replacing one like N313P, or similar, outfitted with higher-performance engines.

Perhaps on a test flight to see how it handles. There are plenty of military landing spots up and down the west coast for a plane like that. An empty plane with big engines could go very fast, and maneuver quickly.

That'd be my best guess. If they had used a shell registration and transponder, the flight would still be tracked and then there'd be a flight record they don't want.

Here's an example of planes that are in this category, and it's not unheard of for a 737 to get souped up engines[1]: https://www.therenditionproject.org.uk/flights/aircraft/inde...

1: Trump put RR engines on his 757, for instance.