Does anyone have an idea what this is under the hood? Thank you!

 

Looks like a grounding nut with nothing attached.Is it internally threaded? Does anything usually attach to it?

My car has the fender bracket, but no shiny piece.

 

If you look, there is another one on the passenger side too

 

Just like @Gio37 posted. It's Nissans way to hold the bolts for the upper control arm.

 

I didn't even think it would be something coming from outside the engine bay. Good catch y'all!

 

One of the first WTF moments when I was doing the wire tuck.
I painted mines because it looked like crap.

I also remembered in my Honda days, the camber kits available would be eccentric UCA bushings and you would set the camber by turning the bolts in the bay.
You'd replace the OEM bushings for these that way your ball joint wouldnt hit the frame.
Would love to see a kit like this for the G.