It is the rear cover of the battery. The front tabs clip into the battery box but not sure about rear tabs.
Scroll to 6.10 in this video.
You can try but I am not sure that you can replace it without removing the cowls as in the video which can be a pig of a job. Looks like the previous owner forgot to replace it when accessing the battery and just left it in the engine bay.
If you bought the car from a dealer I would take it back and complain and get them to remove the cowls etc. and put it back if that is required.
It involves removing the wipers which can create problems when replacing them so they synchronize properly.
Maybe not a dealer's fault. I had been doing some cleaning, vacuuming, with doors open, etc. Next morning it wouldn't start, dead battery. A neighbor gave me a jump; I removed the front battery cover to connect the cables, not paying attention to the negative terminal on the fenderwell. So it could very well be I disloged the rear cover myself. I'm so used to my Honda Element that's not so sensitive to electronics and such.