Curiouser and curiouser.

In the fourth book, chapter 25, Harry dropped the champion's egg and got stuck in the trick step. Moody tried to cover for him, but Snape knew.

"Potter!" Snape snarled, and he actually turned his head and stared right at the place where Harry was, as though he could suddenly see him. "That egg is Potter's egg. That piece of parchment belongs to Potter. I have seen it before, I recognize it! Potter is here! Potter, in his Invisibility Cloak!"

The first time through, Snape seemed to figure it out and guess that Harry was there. But after we learned about Legilimency, that section seems to show that Snape knew for a fact that Harry was there.

On the night of Dumbledore's "Murder," I assumed that Snape knew that Harry was present on the tower but, because the whole thing was a fraud, Snape pretended to not notice Harry.

Now I'm re-reading book 6, end of chapter 15, and I'm not so sure any more. During Slughorn's party, Snape and Draco step away for a private conversation and Harry evesdrops. When they leave, they walk right past him.

"Hardly daring to breathe, Harry remained crouched down as Snape emerged slowly from the classroom. His expresion unfathomable, he returned to the party. Harry remained on the floor, hidden beneath the cloak, his mind racing."

So, can Snape sense Harry's presence or not?