it is the oil sump sensor
it is a seperate sensor to that of the oil pressure switch usually located near the oil pump
this sensor is hooked up to ya trip computer and will say you have a low level of oil if the oil level drops below that sensor, very similiar to your coolant sensor located in the expansion tank
vxhall started using these sensors back in 1992 on the mrk3 astra gsi's as well as other cars