Help identify a part that fell out of MBP

I bought an 8GB RAM kit for my 15" MacBook Pro (2.53 GHz / "Mid 2009") and sat down to install it this morning. Since this was my first time accessing the inside of the machine, when I took off the back cover there was a bit of dust/build-up in some areas. So before switching out the RAM I grabbed a duster and some compressed air and brought it into the bathroom to clean it off.

As I began gently dusting it off into the tub/shower area, two small pieces fell out -- one of which went into the drain. A photo of the other piece is below. It looks like it's 1/4 of "something", and I'm assuming the other 3/4 is what fell into the drain. Since they fell out so easily, I assume it must have broken a while ago. I think the pieces were either resting on top of the battery (under a dust bunny perhaps) or just off to the side of it. (Of course, the pieces could have originated from a totally different area.)

If anyone is familiar with the interior of a MacBook Pro and can help identify this, basically I just want to know what it is, what it does and whether or not I need to get a new one. I did install the new RAM and so far everything seems to be working fine (and blazingly fast), but I just want to cover my bases.

Thanks in advance for your insight.

(By the way, although it may look like a button from a coat/sweater, it's much smaller and thinner. I added the ruler & penny to the pic to help show scale, but even that doesn't convey just how small it is.)