It is a TPMS (Tyre Pressure Monitoring System) "initiator". It receives electronic signals from the TPMS sensor in the wheel and transmits those signals. Two on the front of the car, one each side (duh!) and they fit into slots in the front plastic undertray. To remove the undertray (for whatever reason) they need to be pulled out of those slots and quite often whoever pulled them out forgets to put them back again. Drop the front undertray and you will see the empty slot for the pictured one, not difficult just time consuming due to the number of bolts and screws that need to be removed and then refitted.