[basic.lookup.unqual] p1 says

A using-directive is active in a scope S at a program point P if it precedes P...

[basic.lookup.general] p2 says

A program point P is said to follow any declaration in the same translation unit whose locus ([basic.scope.pdecl]) is before P.

A declaration X precedes a program point P in a translation unit L if P follows X

Hence, whether a using-directive is active depends on whether its locus is before P. So, what's the locus of a using-directive? We do not find the clue in [basic.scope.pdecl]. We should add the omitted part about the locus of using-directives

The locus of a using-directive is immediately after the using-directive.