Where Is The 12V Battery On A Gen5 HEV?

3rd gen battery is under the hatch floor.

The pic you attached (from gen 5 owner's manual?) is highlighting the jump start point. The aforementioned stud-and-nut that feeds 12 volt into the fusebox is elsewhere in that pic. Have a look at the video I posted. Again, the video is of gen 3 underhood fuse box, but gen 5 may be similar.

For an onboard charger running the 120 volt power cord out through the front grill seems sensible. Parked outside, you're able to leave the hood latched, are just running an extension cord out to it.

If on the other hand you were running a quick connect out to an external charger, there could be disastrous vandalism: it's basically a two-pronged connector, with the prongs connected to neg and pos battery posts. Someone putting a jumper wire between the two pins would short the battery. Even heavy rain might short it.