Lupin knew who was alive? I know he said he seen walking out of Hagrid's with Peter. Like Demelza said the map is big with different folds maybe the forest area was folded in so he wouldn't have seen 'them' in forest.

If he seen Buckbeak was alive I don't think he'd care too much or even know that Buckbeak was the Hippogriff who was meant to be executed.

If he seen Sirus was alive - being he was old friends with wanted to see what he had to say for himself on what he was accused with those crimes.