“Snape was trying to save me?”
“Of course,” said Quirrell coolly. “Why do you think he wanted to referee your next match? He was trying to make sure I didn’t do it again. Funny, really… he needn’t have bothered. I couldn’t do anything with Dumbledore watching. All the other teachers thought Snape was trying to stop Gryffindor from winning, he did make himself unpopular… and what a waste of time, when after all that, I’m going to kill you tonight.”
Quirrell snapped his fingers. Ropes sprang out of thin air and wrapped themselves tightly around Harry.
“You’re too nosy to live, Potter. Scurrying around the school on Halloween like that, for all I knew you’d seen me coming to look at what was guarding the Stone.”
“You let the troll in?”
(Harry Potter and the Sorcerer’s Stone)
Judging by this answer of StoneyB’s, the clause seems to say Quirrell didn’t know whether Harry had seen him on Halloween and it wasn’t important to him at the time, rather than now. Is it for this reason that he uses the past tense instead of ‘for all I know’?
Yes, you’re right. He says “For all I knew” because he’s relating what his feelings were at the time Harry was scurrying round the school on Hallowe’en, not what his feelings are now.