Very possibly part of the engine bay plastic undertray.Not very important but I would suggest you get the car up in the air to check its security to the underbody,just incase it is flapping around.

The front under cover should be easily checked by looking under, if its damaged at all remove it entirely, once up to speed that damaged cover can be pulled down to scrape on the road as many a Golf owner can testify even if its sligtly loose.

Some cars have a plastic box type cover over fuel filters/pumps and the like, often situated at the front of the rear subframe and now and again in the transmission tunnel which in wrong wheel drive cars hosts the exhaust.