She’d have been contacted at 18 if it was in her name, I think. She should have been contacted around 16/17 with documentation, according to Google.
It depends on the account. I set up a savings account for my niece when she was a baby and paid a small amount in to it every month till she turned 18 last year. Then I closed the account and transferred the money to her. It was a surprise to her, as the bank certainly hadn't contacted her at all. (I, the account and the niece are all in the UK.)
It was also apparently a surprise to her parents, who had forgotten I'd said I'd do this all those years ago, and forgotten they'd given me a copy of her birth certificate for the purpose. If they had remembered, and had asked me about it at any point, I wouldn't have minded at all - BUT I have a good relationship with both my sibling and sibling's partner, so that's a very different situation from yours.
I don't think it is at all 'grabby' of you to wonder about this, but in the circumstances I can see it would be awkward to ask! Really it should be for your husband to ask, if he is on reasonable terms with his mother. If he isn't, you'll probably just have to let it go - depending on the implications for your DD's tax return, of course. So first you need to check the relevant rules in the country where you live. If your DD might indeed be in trouble for not mentioning such an account - IF it exists - then your husband needs to talk to his mother about it.