The TVTropes Trope Finder is where you can come to ask questions like "Do we have this one?" and "What's the trope about...?" Trying to rediscover a long lost show or other medium but need a little help? Head to Media Finder and try your luck there. Want to propose a new trope? You should be over at You Know, That Thing Where.
Has the feel of Trap Is the Only Option if the detective admits it's suspicious but think it's worth the risk.
... or just Lured into a Trap.
Sounds like Come Alone to me. Searching needs a somewhat outside-the-box approach - you don't know the name of the trope, so you need to work with your associations. Try typing phrases that evoke the trope for you, or have a look through pages for works you're reasonably sure would contain it. Indices are also helpful - this particular trope is only in Stock Phrases, but if you're thinking of something associated with detectives, you can check Film Noir, Cops and Detectives or Mystery Tropes.
You're a detective. You've made some progress, and angered some potential suspects, but you're still basically stuck on actually solving the case. Then you get an anonymous phone call, and a gruff, unknown voice says they have some information they're sure you'll be interested in, and all you have to do is meet them alone, late at night, down on the docks or in some utterly deserted part of town where there'll be no one to see when you get struck over the head from behind in the dark. But of *course* you go, and of *course* this is exactly what happens.
I'm sure this is a well-known trope and on the site somewhere, but I can't figure out how to search for it.