Is there an incidence of a Hiden (Meaning Secret) Jutsu being used by someone OUTSIDE the clan?
Neji learned the Eight Trigrams Sixty Four Palms and Eight Trigrams Heavenly Rotation techniques through his own genius and skill - these techniques were not taught to branch members thus he would never have been taught them.

Ref: http://naruto.wikia.com/wiki/Eight_Trigrams_Sixty-Four_Palms

Edit: though not in the spirit of the question, technically Hashirama's Deep Forest Creation technique was his Hiden jutsu, but after his death was used by Madara after gaining Hashirama cells without any training.

Ref:http://naruto.wikia.com/wiki/Wood_Release_Secret_Technique:_Nativity_of_a_World_of_Trees

Also, in the Boruto movie Momoshiki can absorb and then use Hiden techniques