An NPC that isn't cagey or quirky, answers their questions without prevarication, and is actually helpful to the PCs.
The players will be sure to think something is up.
In a campaign aborted because people moved I had it begin in an inn the PCs were staying at in the Foreign Quarter. The person who approached was neither hooded nor mysterious. He also offered no money to hire them for an assignment. He answered all of their questions as best he could. It was a basic "find this missing person" (an uncle wanting to find his missing nephew).
The PCs were all suspicious.