There is another similar question to this ( titled mysterious beeps) that I just posted this on. It may or may not apply to you also ? https://vwcaliforniaclub.com/threads/mysterious-beep.38582/
Check you roof is properly closed ( which it probably is )
In hot climates when the temperature is raised we get a single faint beep and our display for the roof starts flashing. For ages we were saying " whats that beep"
It never happens in the UK...only when we are abroad in abnormally sustained hot climates.
Scary when it firsts happens but we are now used to it happening and is now just an annoyance and we turn the radio up : )
When investigated it I was told that early cali's ( ours is 2007 ) had sensitive/faulty roof ram sensors.
This was several years ago and my memory is not as good as it was.
Maybe you can find the thread and read what other people have said about it.
I seem to remember that some people had the sensors/rams changed ( damned expensive to rectify for something that affects us personally for 1% of our journeys) and some tried methods to try to mitigate the heat getting to the sensor but probably best look it up yourself as my memory of it may be incorrect or there maybe more updated information.
There is a secondary warning noise for the roof and you will know about it if it goes off as it's not a discrete faint beep and scares the life out of you if going down an autoroute at 60mph. It's time to pull over if that warning sounds and get the straps out.
I'm not an expert and I'm not saying the beep in your cali is the roof sensor for definite just giving you the benefit of what we experienced and an opportunity to check out the available info for yourself should you think it relevant.
All the best.