In S01E02 of Star Trek: Discovery,
T'Kuvma agreed to a peace dialog and then he rammed his ship into the USS Europa in a cloaked state. But, when his ship went invisible,
why didn't Starfleet notice it and go to red alert?
The Europa was rammed by a different ship under T'Kuvma's command, which was already cloaked.
This had been my impression watching the episode, but as evidence I'll submit the following:
Starfleet Admiral at the Battle at the Binary Stars. He was killed when a Klingon cleeve ship attacked the U.S.S. Europa.
The Klingon ship appears atomized after the Europa self-destructs, but T'Kuvma's flagship is completely intact after this; there's no evidence of damage until the Shenzhou fires on it a few minutes later