It was late enough for the sun not to bother him, and on occasion he did actually have to do his work. So when he returned he felt the pull of immortal, and something else on the wind.
Hades wasn't immune to a Siren's call, but he could ignore it. It was a perk of being an Olympian so he wasn't surprised to find her. Nor to see her with a mortal. It also wasn't his job to protect them, but still. Someone had to. Zeus would just grin, and Poseidon would cheer her on.
So he walked forward and made sure to get in between both the siren and the mortal. He looked at her over his shades and sighed. "why don't you go to the river and leave this one be?"