Question over old custody arrangement

[BritishIdiot]

Ex p has an adult child. The mother walked away from the child as a youngster and ex wasn't awarded full or shared custody of the child, only weekends. I was never told why, even when this was questioned. Is there anyway of finding this out? I have children with this "man" and I'm wondering if there is something I ought to know.

so where was child the rest of the week?

A family got custody

It does sound like something you should know. I don't know how you would find out, but perhaps start with a few gentle questions to family members, even your DSS will know what he was told at the time even if that doesn't give you the whole truth.
It may be a perfectly innocent reason - just didn't feel confident enough as a parent.