If the def wasn't topped up at the last service I doubt you could have driven 6,000 miles before the low def warning?
That happened on my second service and a few days later the low def warning occurred.
I sent it back and watched the tech refill the def.
He had to hook the D to the shop computer which actually displayed the % of the def as it was refilled.
Sent from my Nexus 7 using Bimmer App
2013 E350 BlueTec
2011 VW Jetta TDI