tracing group searching for deceased mother

I've just had a letter from The tracing group searching for my deceased mother. I googled and it suggested it could be from railways pension scheme.

my dad worked on the railways and she continued to receive payments after his death. My mum died in 2020 my dad in 2005.

I informed the bank of my mum's death and they closed the account. I cannot remember if I informed the pension company, I would have assumed that the closed account would not have accepted further payments, there was a small amount of money so it was simply paid into my account, about 12k if I remember. so no probate etc.

now I am worried about what these people want? could it be that there was payments made shortly after my mum.passed and I now owe them money? what if there was overpayment?

I've had serious debt issues in the past but we are OK now, however this is very triggering. I'm scared that I'm going to have to have a financial battle.

should I respond or ignore? I was not a dependent at the time my dad died so I can't imagine I'm owed anything