[MENTION=6669384]Greenfield[/MENTION]
Bartender is not trying to give a false impression of the Assassin's whereabouts. Bartender is not trying to convince Paladin that he doesn't know where Assassin is.
Under your assumption of Sense Motive, it detects everything but the whole, unabashed, unrestricted truth.
For example, lets say that Bartender knows that Assassin is sitting in the bar, at table
#7 , wearing a green scarf and sitting on a rickety chair.
If Bartender says "Assassin is behind you at table
#7 , wearing a green scarf and sitting on a chair.", your assumption of Sense Motive means that he would be "setting off fireworks on the trustworthy scale". He didn't tell the whole truth, and the Paladin would be able to tell that he wasn't being honest or trustworthy. After all, he omitted a detail.
In reality, there are a wide variety of honest, trustworthy answers that Bartender could give. For example, if Bartender actually was Assassin, he could say "I haven't seen him today", or "I hear he's staying in the city". Neither of which are untrustworthy, they're both completely honest, they wouldn't register at all.
It isn't until he is deliberately deceiving Paladin that Sense Motive comes into play. He could say "I think he went to the next town over," or "I hear it's a woman", or "He came in earlier and left, hasn't been back since."
The latter examples suggest that he is not Assassin, which is a deception. The former has no deception whatsoever, it just isn't the whole, unabashed, absolute most detail he could possibly provide.