private detective
PI
private eye
private investigator
operative
shamus
sherlock

Up to about a year ago he was a private detective, but has no work now.

I really didn't understand why he needed a private detective.

Not at all what I thought a private detective's office would look like.

Your mother says you told her he was a private detective?

You don't need a private detective to make a run like that.

What if the only thing she saw in him was an opportunity to play private detective?

Why do you have all the private detectives running around the last couple of days?

She tried to run away once, but he sent a private detective after her.

We had more than our share of private detectives for a while.

You didn't need a private detective to check them out.