[Autumn]

Free Focus seems to trigger on passives that don't seem to even be remotely related to spellthief, namely statuses that belong to passives on other classes that you technically gain from your enemy, triggering it.

Those statuses being:

Any Immunity (Claret Call/Fear/Stun/Knockdown etc.)
Voltiger
Poise
Perfect Poise
Prophlaxys
and probably various statuses obtained from items.

[Neus]

They don't have to be related to spellthief or stolen or anything like that. It applies to any positive status effect you get where the causer is an enemy.