As I said, some pacman frontends use `sudo` implicitly.  As such, you can invoke `yaourt -Syu` (or any other privileged operation) without `sudo`.

In fact, `yaourt` explicitly warns you to not use `sudo` to invoke `yaourt`, because it has to compile packages with root privileges then, which — given the amount of poor build scripts — isn't a particularly good idea.  Without `sudo`, yaourt can build packages with user privileges, and only use `sudo` to eventually install the built package.