private detective
PI
private eye
private investigator
operative
shamus
sherlock

I had him followed for a week by a private investigator.

Then another ten years trying to make a success of being a private investigator.

So I started reading about private investigators and got interested.

In that sense, he is something of a private investigator himself.

"There must be some reason why you're coming to me, rather than a private investigator or the police."

It was also said that the paper made use of private investigators.

When that was done, he looked at the private investigator.

What the police didn't turn up, my private investigator did.

The private investigator was paid for a job with the film.

But a local private investigator was also watching him, and he couldn't understand that.