Coolant temp sensor

That's a pic from on the ground did not climb up to see from top side so there might be better access.

 

So how did it disappear?

 

Yes, the brass plug that is leaking is the same location that Gridaxe posted and circled in red. Make sure to check under the cooler that the sensor is not there like in the short video where the person is changing the sensor. So, if you were to put a sensor in place of the leaking brass plug there is enough wire for the connector to plug into the sensor? As I said before there is usually just enough wire to reach where a connector plugs into the correct sensor.
Did this happen after someone worked on the truck? If someone disconnected it there would have been a code on the dash right away. Or someone removed the sensor and jumpered the connector. Or someone needed a sensor and stole yours.

 

Its amazing the quality work that comes out of some of these places. Truck shouldn't have even left the bay with a new, active code related to recent work and a leak.

 

From what I can tell they look identical to me. Whomever worked on your truck should not be allowed to turn a wrench on another truck period. Heck I'm just a shade tree mechanic and I can do better work than that using a list of codes and YouTube.