zedebee

Can't answer your final question. However I think it is probable that both batteries may need to be replaced.  Assuming they are not being drained by something else on your car. 

Recently I replaced mine when main battery was at 12.2 and auxillary at 9.6. Both original from 2014 and at 9 years of age I was happy to renew!

Age of your batteries should give you a further indicator.

Assuming the dealer did an Oil change I would take the car back to the dealer immediately and ask them to check it for you. It could be the dealer forgot to reset the guage. (If this is needed on your model) Hopefully the dealer did refill with Oil!

Can someone please tell me where it is located?

I have loooked behind the trim panel, around the  battery and spare wheel and behind the rear offside wheel without success. I understand that it is part no. C2C 28124 and so I know what it looks like!

My Ancel diagnosed fault. Showed outputs from 3 all within range of normal. Output from right rear sensor was '0'.