It's probably the nut for the yellow screw of step 13. It sometimes comes off the motherboard, with no adverse effects. Just put it back, under the small u-shaped metal shielding

On the Nexus 5 I just tried to fix, the adhesive is pretty solid ! I did puncture the battery. It is now dangerous and the battery should be replaced with a new one.

This part is very delicate. The adhesive is pretty solid and I did puncture the battery. It is dangerous and the battery should be replaced with a new one.

This screw is supposed to tie into a very small nut that sits under a small u-shaped tab in the EMI shield that is fixed to the PCB. Maybe that nut has moved or got lost. I recommend you to lift that small tab to find it because if loose it could create a short-circuit.

But that screw/nut is absolutely not essential to the LCD. It should work without it.

Be carefull of a very small component soldered near the yellow connector. It can pop off and make your front camera irresponsive.