Go down and snag it. It's a storm drain, so there aren't really very many nasties down there beyond the occasional family of cockroaches. I'm a surveyor and here in so cal, we regularly "dip" storm drains and sewers. When they're relatively dry, I have no qualms with climbing down to get dimensions. it's not as bad as you think it's gonna be.
About 4 months ago, my wife and I were visiting my sister's family. Getting out of the car, my wife dropped one of our cameras down a storm drain catch basin. Luckily I had my work boots and gloves in my trunk. I went inside and grabbed a shovel, came back out and pried it open. This was a pretty deep one, about 12-16 feet or so. Climbed down, grabbed the camera (in its case luckily), and came back up. The camera actually popped apart at the seams from the drop, but it snapped back together and it's fine to this day. There was a little bit of water but not enough to soak through the case.
Anyway, just head on down and check it out. If it's got a decent amount of water in it, just put on some waders or galoshes. You'll be fine. Good luck.
As a kid we walked through many a storm drain. Get a buddy, pop the cover, and retrieve your treasure.
But be careful, and let us know the outcome.
