I see this all the times and is my pet peeve. Thing is you can’t flash them as by time you notice they’re behind you
I was a guilty driver when I first had a car with switchable running lights (Alfa). There was no information in the manual explaining their use and did not say that though the front lights were on the rears were not. In dusky conditions I was happily driving along and wondering why I was being flashed ... as far as I knew my lights were on ... but of course I subsequently discovered they weren't! Embarrassing and dangerous.