The prime minister, who had just done a viewer call-in on ITV's This Morning, made the comment as host Phillip Schofield moved on to the next item.
Schofield said: "Up next, a man who can pinch your wallet, your watch and even your tie without you noticing."
Cameron, who was off camera, could then be heard saying: "Is that Alex Salmond?"
Co-host Amanda Holden burst out laughing at the remark as the programme went to an advert break.
With just 15 days to go until election day, the campaign has focused on what role the SNP will play in a hung parliament. Salmond, who is hoping to return to the Westminster parliament as MP for Gordon, said last night that the party had a "broad smile".
"We know we've got two weeks to work really hard to maximise that positivity we're getting in the communities and on the doorstep, and turn it into a real result for Scotland and that's what we intend to do," he told an election hustings.