Coolant temp sensor

No clue either but most coolant temp sensors I've seen are in or near the thermostat housing, typically front corner of the head somewhere.

 

Looked up temp sensor to see what it looked like and what I found on my 16 DD13 that looked the same is right behind the fuel filter.

 

Nevermind that's the manifold temp sensor. Will update if I can find it.

 

Detroit DD13/15/16 I6 Engine Sensor Locations – TroubleCodes.net
Correct, front of engine at EGR cooler.

Volvo D13 coolant temperature sensor change - YouTube

This is what I googled. Sometimes takes several tries with different wording. Took about 5 minutes.
location of coolant temp sensor on 2016 DD13

 

Here's #11 on mine it come's straight out of the front, pretty tight in there.

 

You say there is a wire not attached. I take it you mean a connector. If so, there is not much extra wiring where a (any) connector plugs into a sensor. So the sensor (or a plug in place of the sensor, though I find it hard to believe someone would be that stupid) has to be near the connector when the wire is extended out. If you can't find the sensor or a plug then I suggest removing the air cleaner. There is the location in the short video I sent and the location that Gridaxe posted in post #7. Do you see a plug in either of the 2 locations posted? May need a telescoping inspection mirror.