DEF Fault Codes - P2003A or 004BAC - when tanks are pretty full

[FunfDreisig]

Over the last six months our 2009 X535d w/80,000mi has thrown 3 DEF fault codes when the DEF tanks were not very low.

1 - Last summer it threw P203A: It had been awhile since I added DEF so I topped up the Active tank, cleared the code and added DEF to my shopping list.

2 - Nov 11th it threw another P203A: I filled both the Active and Passive tanks and cleared the code. But was surprised that neither one took as much DEF as I expected.

3 - Jan 11th it threw 004BAC (via Carley): So I topped up the Active tank which only took a quart!

Thinking back, I noticed the last two Engine lights (DEF codes) just after turning off a main road and down a hill. Has anyone else has seen DEF codes when their DEF tanks were pretty full? If so, did the engine light come on during/after a quick turn and/or change of slope?

Funf Dreisg