I'm guessing it might be a sensor of some kind. It was hanging loose on its wire down the back of the engine on the near side (beneath the tandem pump).

Car is a 2008 Roomster Scout 1.9 TDI

 

Many thanks in advance.

Dummy speed sensor. Basically your gearbox doesn't use a mechanical speed sensor, so that bungs up the hole instead and gives the unused loom connector somewhere to go.

Should just clip back on, top of differential near right/offside drive flange.

ABS sensors now do speed sensing.