Now I love Eleven and all of the cast, but theories have been going around that there's more of Elevens kind, people with the same powers (maybe even stronger according to some people) and that they're in hiding from the government, just like Eleven. That they escaped before her, and she's number Eleven of the ones with special powers. If the government and 'bad men', as called by the kids, assumed the person to be dead or they've spent too long looking, they would find more people with powers like them or replace them. Kind of a weird theory, I know, but assuming the way the government figures are, they would definitely be able to find the missing 'science experiment' or replace them with someone else, and it'd simply be a never ending loop of experiments until the person escapes and they can not find them.