Oh I didn't think of that--that Vin was spiked. (Sorry, I would quote but the computer I'm on is being stupid). That could very well have affected the situation.
When I first read that part, I thought it was a trap, too. That would explain why Vin turned around. But now I do think it was Hoid. The man was in the square Cett had told her he'd be in, plus he was whistling, which is what Cett told her he'd probably be doing....It would be way too coincidental for it to have been a trap for her. There wasn't really any way for the people in Fadrex to find out that it was Vin coming, and she was coming that night, and that Cett would've told her about that particular informant.
And I wasn't saying it would be her metals giving away his power. I mean intuition and instincts. Vin runs on both of those--more instincts, though. It could've just been that she sensed (not with allomancy, but simple skills that most humans have) that something was not quite right about the person whistling. Or it could've been Ruin whispering to her. I like that theory too.