I dropped my lower splash guard to access the SC water pump, and these 2 guys are just hanging out. Where do they go? What are they?
2010 xf 5.0sc us version

 

Those are your antennae for the TPMS. They fit into little grooves in the plastic engine splash shield.

 

Yep.

This should be a sticky, that question gets asked at least twice a year every year, along with "my interior lights no longer come on when I open the door, help!" (and the answer of "reset the auto lights on by touching the centre light for two to four seconds until the lights flash twice") and "what's the little cable/wire and plug that's not connected to anything running off the battery for?" (and the answer of "for dealer pre-delivery set up, never used or needed for anything again, nothing to worry about") and "my headlights are pointing way too low how do I adjust them?".
Seriously, maybe admin can create a sticky thread for "commonly asked questions" such as these.

 

I feel this image from the service manual best explains how they go back in.