Balloons

designed for highest altitudes (which are outdated by

space satellites now) were not closed, but open at

the lower end.

They get a small fill of hydrogen/helium  before launch, which 

will expanded to fill the balloon at design altitude. 

Any excess would blow out.

Such balloons are recoverd if the payload is expensive , but if

one had lefts them float, they will loose

their gas fill  slowly by diffusion, sinking back to

ground within days/weeks.

Georg