Do you recognize this part?

That is part of the engine under cover. You don't need to be worried that it no longer is attached; however you should check under the car to see if the remainder of the under cover is secure - or if it is about to fall off also.

 

Patrick is right.
This part came off my car when driving across 260km of very rough dirt road in Queensland. Well it didn't fall off it came half off and was hanging down just behind the front bumper when I stopped in the first town off the dirt road. Leave it off, put it back, makes no difference you can measure.

 

Mine fell off and I had it reattached the next time I took it in for oil service. Most of the under engine covers are to keep rocks and dust off the engine. It's not necessary but it does have a purpose.