That part is so tiny that it's hard to be sure, but I'm thinking it could be this part.
[image|3064552]
It's from Step 22 of the Pixel 5 Motherboard Replacement guide here.
[guide|140529|Google Pixel 5 Motherboard Replacement - iFixit Repair Guide|stepid=280352|new_window=true]
Step 39 shows you how it goes back in.
[guide|140529|Google Pixel 5 Motherboard Replacement - iFixit Repair Guide|stepid=280373|new_window=true]
Hopefully that answers that! Unfortunately, it looks like you may have to remove the motherboard to get the darn thing back in where it belongs.