A
Welcome to Android Central! You don't remember the PIN you presumably set? Or is this someone else's phone?
They didn't necessarily set one... I had this happen on my S10e. I definitely didn't set a PIN on the volume limit, but one did randomly appear once.
Don't know if it's a bug that hasn't been fixed or what.
That's always disturbing. I wonder if there's some default PIN that the user is supposed to know about.