under the couch/cushions, in the car (changing after practices), at a sleepover, in the garage/mudroom, in forgotten quick-change bags, left in luggage, tucked in unused shoes, at the sporting center...
my toddlers take their socks all over, in cars, probably at the park with my husband, in every room in the house, they are never together!! I used to keep them all in a ziplock bag and hope the match showed up but now I just dump their single socks in their drawers and from time to time I match up what I can, otherwise .. they wear mismatched socks. And I've made my peace with that
When they go to school they will only be able to wear white socks so that will take care of that problem!