Fluid stashed in hatch leaked into battery?

I got a "Check Hybrid System" warning in my 2011 Prius while driving home. No other warning lights, and the car seems to be running normally. I have turned it off and started it right back up with no trouble other than the warning light.

My usual mechanic doesn't do hybrid system stuff, so I took it to the Toyota dealership. Again, no issues with driveability.

Toyota guy says a bottle of brake fluid stashed in the hatch bin has leaked onto the battery. Now, there was indeed a bottle of brake fluid stored in there, and I can certainly believe that it may have leaked. I'm having a harder time imagining how it made its way to the hybrid battery.....
Toyota guy says pricing to take out the battery and clean it up will *start* at $800!! He also says it isn't safe to drive the car, but that may be a tactic to ensure that I agree to let them do the repair. He also couldn't tell me what codes it was throwing-said he didnt have that info in front of him (phone convo)

What on earth should I do to avoid being taken for a ride by the dealership? Maybe they're on the up and up, but I am def. operating from a place of skepticism. And I DEFINITELY don't have an extra $800+ to burn unnecesarily. What questions do I need to ask? Does $800 seem reasonable for that kind of work?