The B777 is not like any other Boeing because of the SAARU. When you turn off the IRU (There is only one, but it has lots of gyros in it) the SAARU, or standby IRU, keeps running, powering the Standby Horizon, like on any other aircraft, but on the B777 its output is also fed to the main AHs. To get all yellow flags on them you must switch off the SAARU as well, which can only be done by tripping the CB.