Clerk told me once...to make room for gluten-free foods, other things have to go. Need shelf space.
Did they move the salt to the baking isle? I see it there sometimes too.
I'd bring it to the attention of the grocery store. They'll either tell you where it is, or dropped a product that is more in demand than they think.
The good news is, when you do finally find it, you won't need another box for a year! Mine seems to last forever....