Shisui is dead and has no real relevance to the series at the moment tbh. In fact, in the manga all he was is a plot device in order to show how Itachi got his MS and to emphasize how difficult the situation was & how the Uchiha did not want to negotiate, so something on the level of a KA was necessary. Considering that the 4 main Uchiha (Madara, Itachi, Obito, Sasuke) are all more powerful than he is, and that Kishimoto is done writing about the Uchiha massacre, he's not showing up ever again.
Shisui wasn't a part of Anbu, considering that in the manga it states that Itachi was the only Uchiha in Anbu. They found Shisui's corpse missing both eyes. They probably incinerated his body and that's why it's never been found. After all, Itachi did say in the manga that after Shisui gave him his eye, he left and died silently (jumped off the cliff). He's gone.