The thing is, spares at her house is a loving thought for a son. I think that's quite sweet, really.
I do sigh a bit when people grumble about oddities with MIL that are just part of normal human ranges, especially when clearly motivated by love. My own MIL is a really troubled individual and DH wanted us to keep her at arm's length because he doesn't trust her as far as he can throw her.
Your MIL hasn't done anything but try to provide for her son. It's not critical of you, or about you at all, is it? Underwear at her house, which you didn't even know she had, can't be anything but love. She's just his mum, and wants to provide for him in any way she can, which doesn't clip his wings or threaten his independence. You'll probably do the same, albeit perhaps not pants related, one day.
I am excited by the idea that my kids may one day be independent, because at least one (dd is too small to know, as yet) has some SEN. But I also know that however delighted I'll be if he does form his own completely independent life, I will miss him dreadfully from the house, and will want to provide loving nurture when he does visit. It's what a mother does, I think, if she can. So I understand her instinct... just as I understand that it seems barking to you! 