Depends what's meant by the question.
'Responsible' as regards who did the deed, who knows? But, as Chris says, the suspect list is small, and police best placed to investigate. Assuming they're interested.
But, as Edders alludes, 'responsible' has a wider meaning in respect of contractual liabilities, or maybe in terms of a duty of care regarding customers' property while it's entrusted to their possession.
So Sussex says: "Whoever had the keys when the stuff went missing is responsible IMO."
Does he mean that because they had the keys then they were prime suspects with regard to criminality and theft?
Or because they had the keys they were responsible for the safekeeping of the DPF while entrusted to their possession by Grandad, so there could be some kind of contractual responsibility if a third party was responsible for the criminal act of theft?
I mean, there's a possibility the DPF was simply mislaid, so there's no criminal act, and only contractual.
But if it was stolen, then there's a criminal act and possibly a contractual failure. But even if it was stolen, there's not necessarily any contractual liability if those entrusted with the keys and possession of the DPF did all they reasonably could to safeguard the parts.
