The sensors were only picking up life signs in the Saucer section. However they couldn't confirm those readings were true.
We do know that Main Engineering was completely empty. So we can assume everyone died there. Heck everyone on that deck probably died considering how empty Main Engineering was. Logic would state that those that survived would go to Engineering to organize and figure out what is going on.
However, the thing that has ALWAYS bothered me about the episode is the sudden end. Ship saved, Keiko has baby and then BANG next scene shows the ship at warp and everyone is all happy. There is absolutely no follow up on the damage, repair efforts or death toll.