I found my sigg! It was in the lost-and-found at the climbing gym. Covered in chalk, but otherwise in fine condition. I was so excited when I found it that the people working there gave me a funny look (they all know me pretty well, so they weren't surprised by my enthusiasm).
Ha! I came on to this thread to say that the yoga studio where I teach has a giant Lost And Found tub in the back, which is pretty much 100% water bottles, and a few pairs of shoes. So, I guess, if you ever lose your water bottle, be sure to check your yoga studio. :)