Stainless Steel Devil Rat's first post is accurate.
Generally speaking, "possession" is a very vague term. You are right catrone3, that it's something related to having played older editions, but it's also a bit of contextual understanding you get about the game as you play and read the books. I'll bring this passage to the attention of the errata team, as a simple change from:
"For a focus to remain active, it must be in the possession of the magician (worn, carried, hand-held, in a pocket or pouch, etc)."
to
"For a focus to remain active, it must be in contact with a magician's aura (worn, hand-held, or else hanging close to their body)."
Would go a long way to make things clearer for players and GMs, while also giving insight to the nature of magic in the setting. One of these dozens of magic books should really include a direct lexicon page where things like Essence, Aura, Signature, Mana, and so forth are all given clear definitions. Even just including it in a 12-page PDF-only book would go a long way...