There are many people that received a few pounds of bonds as christening gifts years ago and when they pass, it's possible they've been forgotten when it comes to probate and letting NSI know they've deceased.
When Mum passed we let NSI know and they allowed Dad to keep the bonds for 12m before automatically stopping their prize winning powers. Probate came through a little later and Dad had the money released.
So, I don't think prize errors will be made generally when.people pass only when NSI not told which, given how few bonds many people.hold, that's unsurprising.
Mum had £13 worth and Dad's only record is scrappy little counterfoils with no names on them.