He can sense people at pretty far distances, and the battle is at Hogwarts, so he won't have far to go.
Yeah, Voldemort was working in secret when he and the Death Eater openly attacked everyone at the school.
Only Harry has the invisibility cloak, why would he give it to Voldy? I don't care if he was on my team, the guy who killed my parents, and is responsible in part for the death of my godfather, get's no help.
Why would he be off his guard or asleep in the middle of a huge fight?