The sensor (circled in red) is located below the coolant tank and is in a metal elbow. The sensor snaps into place (not threaded) and has a single o-ring seal which coolant is leaking past.

I believe this is the coolant temp sensor?

I checked the fische on RealOEM.com and cannot find this sensor listed and the coolant lines do not match what is actually on the car.

 

I found the fix over in the 'fixing problems and issues' subforum. Looks like a well known problem and I could not find the OEM parts listed as Mini/BMW has did away with this problematic setup.

https://www.northamericanmotoring.co...ntryman-s.html