I doubt it's radiation detection devices... they're passive and don't emit anything.
Radar was my first thought, also. Next time you're there, the way to check for radar is to turn on your radio and listen for a short, harsh buzz that repeats itself every 10 secs or so. I used to hear it when I worked at Alameda NAS.
Never been to S. California. Is there a base nearby at the border?
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