It's not clear if the Lars Moisture Farm hit is made to look like Tusken slaughter or not. That would get more scrutiny than dead Jawas.

Arguably, both Artoo and Obi Wan have reasons to pretend they don't know each other from the old days. Threepio doesn't remember, and Luke would ask more questions than Ben has white lies. The Wars of The Stars edit did address this though. ;)

It's been speculated Vader prevents Boba from shooting Chewie in ESB because Threepio is on his back, but that's a stretch. A Dark Horse comic does have Vader recognize him, and he gives the order to have the blasted droid put in the cell with Chewie.

I have wondered if Artoo can see Obi Wan's ghost...