NS&I letters - lost account?

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My older brother had and account with NS&I opened for him as a child and the passbook was handed to him at 18. My younger brother had a similar account which he managed to get the details together for a closure cheque.
In 2013 they sent a letter addressed to my uncle (house in the family since 1950s, he last lived there 30+ years previously, now lives abroad) which had an account number on it. My mum forwarded it to me by accident, I emailed my uncle the details and he generously gave me 10% for my legwork.
Last week they sent a letter addressed to my late dad regarding interest rate reductions ‘This will affect you if you have one or more of these accounts’: Investment Account, Direct ISA, Direct Saver, Income Bonds. It doesn’t quote any account number(s). My mum forwarded it to me (my dad had the same first initial and I question her reading skills). I’ve got the PDF for tracing lost accounts printed for her to fill in next time I’m home. The will, death certificate, etc aren’t going to be a problem, but it’s weird that it’s the first letter from them since he died in the late 1980s.
Yesterday, my mum told me that another letter had arrived from the same Glasgow postcode (addressed to me this time, really!) Despite her telling me when I turned 18 there was no account in my name, I’ll await the letter and go through the tracing procedure.
Has anybody else experienced this weirdness from NS&I? Have they discovered an old computer system recently and decided to send out letters when they never bothered before? I suppose we are lucky in having the same address all along.