2015 CX5 and the battery died; after recharging to 100% we started the car and it showed a quarter tank of fuel (after just topping up).

Thinking maybe the car was left on by someone on accident and the fuel just idled away, we left for a gas station to refuel only to find the tank was already full.

Does this sound like a fuel sensor issue or is this electronics that need to be reset after a dead battery? Or something else entirely?

Any helpful comments appreciated.

 

A dead battery never fully recovers, so don’t assume that all is okay. The issue could be due to low voltage under load - a common problem with most modern vehicles. OTOH it could be a sensor issue. The first step is to have the battery electronically load tested, most large chain auto stores will do this for free. If the battery checks out okay (get the actual print out!) then the problem might be a sensor.