The AdBlue additive is in a tank under the boot, and is topped up through a filler cap in the boot itself.
According to the manual, it can hold 17 litres, enough for 12,500 miles of driving. If the level falls to below enough for 1,500 miles the warning light comes on. If it runs out the engine won't start at all.
The manual also says if it's a temporary fault, the alertdisappears during the next journey,after a self diagnosis test of the SCRemissions control system. That might be what's happening with yours?
I thought much the same thing as I was typing that... I've only had the car with that engine for 3 months and I suspect I'll be clocking up 12,500 miles well before the annual service is due. Seems it's around £30 for 20 litres on Amazon which isn't too bad if you're prepared to do it yourself.