That looks like the Air Charge Temperature (ACT) sensor in the plenum chamber: connects with a plug in the engine wiring loom, with brown and brown/yellow wires going to ECU pin 46 and 25 respectively. At least, that's the case on my S3C.
The engines without a catalytic converter have the input air temp sensor built into one of the vane air flow (VAF) meters. The engines with catalytic converters do not have the VAFs, so they need to measure the air temp. by another means. Hence the ACTs. Here is some info.
Pretty sure that's right. The casting on the plenum of my S3 doesn't have a sensor there. Maybe a plenum from a later S3C has been fitted at some point?
