That's the same thing as the "lock blocks" you see in some of the single player levels which appear or disappear when you destroy silver crates. They don't have to look like lock blocks or white outline boxes and in fact can look like anything at all, which is what you're seeing. In multiplayer games, destroying the silver crates will make groups of tiles solid or unsolid for everybody, but there is also a "trigger zone" event which will make a group solid or unsolid only for the person who goes on that tile.