Hybrid battery voltage sensor replacement

After leaving a 2010 Prius pandemic-parked for a few months, and then removing the starter battery to recharge it, the car now starts and drives fine but the Check Hybrid Battery warning is on. So we though it was just case of resetting the ECU because the starter battery had been disconnected, but unfortunately the stealership told us the hybrid battery voltage sensor needs replacing due to corrosion that they are convinced was caused by water, and that it would cost $900+ ($400 parts, plus 3+ hours of labor)

Is this a believable diagnosis? Sadly I never trust dealerships anymore when it comes to honestly and accurately diagnosing problems. First off, everything was fine before the car was laid up for a few months, during which there's no way water could have got inside the car unless there's an external leak somewhere letting in rain, and even then it would have to be a very specific leak to direct water down to the sensor below the trunk. Secondly, how on earth can it take them 3+ hours to install a sensor that they apparently were able to diagnose as corroded without charge. Are there any non-dealer places that work on hybrid cars that might be able to diagnose and perhaps replace the sensor for less?