Sasuke is totally looking at us. Because he can see us, you know. Like the moments in typical anime, for example, when one character can't see the other's face yet somehow knows what sort of expression they have, be it serious, twisted, or, usually, kooky.
Anyway, if you look at the ground well enough you can mistake the mud and some of the rain spots as seperate footprints from Sasuke's. Maybe he's following all the members of the alliance? The sinister smirk can be, like, an indicator. "The fun is about to begin." or "I've got you." or something. Iunno.