Folks, thank you for all your comments. All of you who suggested that the mysterious bong was a personal POI were spot on.
Today, right on cue there was a bong as I passed Strensham services which reminded that I had forgotten to ask Mrs D to film it.
It also reminded me that I hadn’t checked personal POIs, but when I did there it was, albeit as coordinates rather than an address.
I must have stored that location accidentally at some point because I don’t even use personal POI, maybe I should!
Thank you once more, the collective intelligence of MBClub is a remarkable thing, and rarely disappoints.