I believe it's the coolant reservoir for the battery cooling system, but it's a real pain, at least for me, trying to gauge the level being right in front of the radiator towards the driver side. I ended up just adding some again and guestimating the level. Wish they put the level marker on the opposite side of the tank where it is more open/visible!
Gonna try to upload two pics to Google Drive and link them here because I don't see it's possible to upload.
https://drive.google.com/file/d/1xyBjq_ ... p=drivesdk
https://drive.google.com/file/d/1y4mIYA ... p=drivesdk