I've seen that as well, back when I used FileVault for all of a week. It seemed like it was pretty standard. I didn't think it was anything out of the ordinary, just something related to shrinking the size of the .dmg file or some such, but I'm sure someone else out there can provide a much more meaningful description of what's going on from a technical perspective. Anyway, whenever I got it it didn't seem like anything to worry about, it just seemed like the normal activity for FileVault whenever you wanted to shut down.