It takes a bit of detective work at times.

There are some repair issues that wouldn't have been detected in the interior BPO (HVAC condenser, supply line leak under a slab foundation, etc.).

I have a client in Baltimore that says he's been contacted several times by detectives.

CO leaks from gas furnaces are not that common and, as you apparently found out, the newer systems have their own internal CO detection (independent of the stupidity of the tenant).

Whether they do a convincing enough job to avoid detection or question of impropriety or the ability to mount a good defense in the case of a prosecution is a different question. 

In extreme cases we even hire a private detective for $200~

Computer software is affordable and sophisticated enough these days that applicant-supplied documents can be altered by computer - these alterations might be difficult to detect; or the applicant simply used TurboTax or similar to freshly produce something fictitious from scratch just for you.

For someone like me it's called being an opportunity detective (or Bottomfeeding opportunist, if you prefer).

It will take some good detective skills and may require a few bucks to find a phone number.

That seems the most logical since there is no monthly min.Regarding private eyes/detective haven't heard too many recommendations.