A little Schiffer
book I bought at Tankfest has Three pictures of A13 Cruiser IVs in service with this Pz.Abt. 100 (Flamm) unit (looks like they were fitted out as the unit recovery vehicles, strange to think of British manufactured vehicles heading East with the Wehrmacht, still nine of them on the unit strength in June 1941... all gone a month later).
Pretty clear rear & front views seem to show no sign of these circles, and a magnifying glass (tragic eh? ) shows no trace of them anywhere.
It's still speculation really, but until we hear more I'm becoming pretty content that it's a warning to troops behind the vehicle to move with caution around it's area affect weapons.