Possible prohibited transaction

Prior trustee probably committed prohibited transaction but not all facts are in so there is small chance he did not. Has either the IRS or DOL published any guidance as to when prohibited transactions must be reported.

Isn't there always an excise tax due on a PT?  So not reporting it is a failure to pay a tax due isn't it? 

https://www.irs.gov/retirement-plans/retirement-plan-investments-faqs

The 5500 (both regular and SF) require a PT to be reported.  I am not sure if the 5500 EZ requires it.  

The issue we have is that the transaction appears to be for the benefit of the former trustee but not all of the facts are established, though its hard to see how the PT rules are not triggered.

Form 5330 would outline when the penalty and the filing is due.

This is not just an excise tax issue, but also an issue under ERISA of making sure that any losses to the plan that were a consequence of the breach are recovered. The current trustee/fiduciaries has/have a duty to investigate and reach a conclusion as to whether the prior trustee committed a PT, and if they conclude one may have, to confront the prior trustee and demand either an explanation or compensation to the plan. Reporting the PT, or not, and paying excise tax is secondary and will be easier to figure out once you've taken first step of fiduciary investigation.

Alternatively, you could call the DOL and ask it to investigate.