Why didn't Ben recognize threepio?

So what would you have Ben say to Threepio? "Hey old buddy! Howyadoin? So good to see you!" Naaah. He probably knows Threepio's propensity to blather, so Ben purposely kept his mouth shut.

What good would it do to say "Vader built Threepio" or "your father built Threepio, didja know that??" It would blow the whole thing.

Not to mention Kenobi had been at the bar earlier that day, so the parts of his brain controlling long term memory of irrelevant droid-related issues was rather out of commission.

Sand grains in the brain don't help either.