I was given a Varia RTL515 in August and found it to be one of the most useful gadgets to have, when it works!
I've found that the car detection stops working on my Edge 520 after a while, it's not repeatable as in every 45 minutes or half hour. It will just stop giving me notifications.
I am sure the Varia itself is not the issue but the 520 as I've stopped and switched off the Varia and the 520 doesn't give any warning of failed connection and still shows as if the radar is still connected.
My Edge is running 13.20 and the Varia is on 3.26.
I've seen this reported as a problem with the old Varia units too so very disappointed that it's not been fixed as it means the device can't be trusted when it's obvious safety bonuses should make it a vital piece of kit for any cyclist doing a lot of miles.