What is this and how do I put it back?!

I'm not in a position to look, but I'd say go to exactly where you think you found it and look around. Odds are it dropped down rather than flew up.

 

Probably important or it would not be there.

 

There's a good description on how to remove the wipers, wiper motor, and cowl assembly somewhere in a thread on installing the engine block heater, as doing so makes that task easier. As I recall it requires removing several bolts of various sizes and the whole thing lifts off easily after unclipping a wiring harness.

 

There is this excellent youtube video from one of our member here, NutzAboutBolts, where he removed the cowling to replace the spark plug.

I would consider opening up the cowling only as a last resort. If I may suggest a couple things 1) get a camera down in that area and take a lot of pictures where you think it might be, 2) go underneat the car and look up (maybe a flashlight will help).

Also keep in mind the thing might or might not come from the car. It could came from someone fixing something on the car and left it there.

Yesterday I went out to my car at work and look for you but couldn't see anything. I then found this empty hole (pic attached, right where the yellow #66 or 99) on the engine mount right next to the antifree reservoir where I thought that rubber plug might fit. I then looked at Ebay pics of car for sale but none has anything on that hole, so I threw out the idea. I also checked the underside of the hood, but nothing similar.

 

I would think a trip to a Toyota parts department would help you find out what they are and where they go, if anywhere.