Why didn't MG Rover's inspectors call in cops?
It's been bugging me for 48 hours so I have to let it out.
For more than four years a top QC and a leading forensic accountant have been investigating what happened at MG Rover between the moment it was taken over by the Phoenix Four in 2000 and its demise in 2005.
The professional-services meter has been ticking over for more than 1200 working days, so the taxpayer has been presented with a bill of more than £16m in respect of their diligent labours.
For that money - and on the basis of their sparkling CVs - it's not unreasonable to assume that they're better qualified than most to spot prima facie evidence of a crime.
So why, in all those years of probing MG Rover, didn't they call in the Serious Fraud Office - or recommend that the Business Department (which used to be the DTI) bring in the SFO?
Apart from anything else, I am told that the Business Department received very regular updates on the progress of the investigation.
Why was it that only after the Business Department received their finished report on 11 June that the First Secretary, Peter Mandelson, determined that the SFO should be asked to investigate whether there's been a criminal act?
More than that, on a reading of the Proceeds of Crime Act, one of the inspectors - Gervase MacGregor of the accountants BDO Stoy Hayward - would actually have been obliged to bring in the police just as soon as he smelled something untoward.
That said, a senior lawyer tells me there's a convention that POCA is suspended for inspectors - which is a bit odd, but there you go.
Anyway, I assume there is a reasonable explanation for why Mr Mandelson felt it necessary to bring in the SFO, but that Mr MacGregor and his QC colleague, Guy Newey, didn't do so at an earlier date.
But of course it's impossible to work out what that might be, because we're not allowed to see the inspectors' report.
And we're not allowed to see the inspectors' report because Mr Mandelson has passed the case to the SFO.
Which is almost a logical paradox of the kind that's designed to make us go batty if we reflect on it too long.
What does seem unsatisfactory is that taxpayers have spent a colossal sum trying to understand more about why a major employer collapsed and yet we remain none the wiser.
It's worth remembering that the failure of MG Rover represented the end of volume car making by British-owned manufacturers.
MG Rover's demise represented the death knell for an industrial policy of propping up an indigenous car industry that went back decades. Even at the end, this government engaged in frantic, fatuous efforts to prop up the business.
Were MG Rover's final, inglorious death throes the consequence of fraud by the Phoenix Four, the quartet who bought the business (the Phoenix Four deny any wrongdoing)?
Did it stem from their incompetence? Or were they just battling against insuperable economic odds?
And why, in the spring of 2005, did the government provide short-term financial support but withhold more substantial aid?
Until the SFO has completed its inquiries, we're not going to know.