I recently bought a 32GB SD card for my 60D. I have it set to raw + jpg and when I inserted it for the first time it showed 898 shots available, which looked right because my 16GB cards hold approx. 450 shots on a new card with the same settings.
I've taken 114 shots so far and noticed that the available shots were 234.
When I examine the card on a computer the image files add up to 3.3GB but the properties window shows that the used space is 22.4GB, and I can't see any other files on the card, unless they're hidden
Should I just reformat the card?
I had the same issue. It is a possibility that you have indeed some hidden files. To delete them you should format your storage card.
On a canon you can do that in the menu.
Apart from hidden files, it's possible that the filesystem on the SD card has become corrupted. A filesystem keeps a record of all files and what space they occupy on the medium, as well as a record of free space. It's possible to have space unaccounted for (i.e. neither used by a file nor on record as free) and thus "missing". This can happen e.g. when power is cut during a write operation.
To repair this on Windows, you can use the error checking operation in the disk properties dialog: