The seller sent it loose inside a thin Jiffy bag.
Awesome customer service...!
I hope you managed to get your money back for this one.
I don't have a Canon 100mm but I do have a significantly cheaper sigma 105mm 2.8 macro. I too bought it online - and after a few months noticed the tiny sticker with the serial number was missing. (It's not something that simply falls off.) I'd actually bought a couple of lenses from the same seller and they were both missing serial numbers, which led me to believe they were both stolen. Since the seller was no longer on fleabay (surprise surprise) I figured there was not much I could do about it - if I made a fuss I'd most likely end up with no lenses and no money. In the end I didn't make a fuss.
Anyway, the Sigma's a pretty decent lens for static photography. Appallingly bad autofocus so useless for anything that actually moves. Of course I'd prefer a Canon L lens instead but as we know they're rather pricey...
Good luck finding your dream lens! (Did you look at the 70-200 f4? Seems to be great for the money.)