Just now, P-47 Thunderbolt said:Remember "the force is strong with this one?"
I would say that Vader sensed Obi-Wan himself not so much Obi-Wan's force "signature" if you will.
You're referring to the line that Vader said towards the end of the movie, when Luke was alone in his X-Wing (both of his wingmen were out of the picture) and was actively reaching out with the Force. Obi-Wan was merely a muted presence in the Force at that point, and was doing nothing more than coaching Luke from beyond the pale.