The fan can still run if the engine is off, as the feed to the fan is always live so if the temp sensor gets hot enough the fan will run

I've nicked a few of these looms and some have this extra plug and some don't My 90 spec GTI doesn't have it, but the loom I pinched from an 89 GTI did, though it wasn't plugged into anything. I believe this sensor was on the early K-Jet GTIs and also on Rallyes/G60s. OI heard that on the Rallye/G60 the switch turns on the fan if the zaust manifold gets too hot, as they can crack under high temperatures.