Somewhat relevant article by CPW I believe:
Near the bottom:
"Warranty: By the book, you only get warranty in the region your item was originally destined for. If you have a Canon Japan camera, technically you’d have to send your camera back to them for warranty service. In practice though, Canon USA is extremely lenient with its warranty service, especially if you bought the body in the USA. We’ve never had a reader tell us their US-bought camera was rejected for warranty service. Note, the above statement doesn’t apply to clearly imported versions (i.e. different camera name (650D vs. T4i), tampered serial number, etc).
Canon repair centers have stated they can’t figure out what’s import vs not based on serial number. We’ve had a few readers tell us that sending a non-US version body for warranty repair was successful, but not enough overwhelming evidence for us to loudly announce it. At this time we recommend trying to send items in for warranty repair, even if they’re import version; Canon seems to stand behind their products and that’s a good thing! Contrast this with Nikon: Nikon USA repair centers won’t touch a grey market item, even if you’re paying for the repair!
Note, in this situation, warranty is at the discretion of Canon and they can unilaterally change their policies at any time."
I'd ask more on how they determined it was dropped if you know it wasn't. Also, how did Canon know it was gray? Hope things work out though.