Mine does this every year, as in starts doing it. Having done some investigation into its function and how it works, it would seem that it applies with a certain level of tension. It then rechecks this tension periodically, a bit like the suspension checks its levels every six hours. My guess is that during the “sleep” time of the car, the actuator has noticed a reduction in tension and doesn’t believe it’s correctly applied, then flashes the warning when you start the ignition.

Why does it lose tension? No idea. I will say though, that when I see this, if I perform a clean and reset of the EPB e.g. perform the drum adjustments as per the brake bible, the errors stop happening.
Whether I’m guessing right or not I’m not sure, but my experience shows that when that error starts happening, if I action an EPB adjustment it’s good for another year