Same thing has just happened to me! Driving in heavy rain on the dual carriageway, got the same message. No loss of power, driving fine. Also a 2015 A3 E-tron. Currently waiting for RAC to check it over, hoping I can at least get it home as I’m 30 miles away! I’ve got a really good local independent mechanic who works with EVs and hybrids so hopefully he can sort it.