When something is "in someone else's possession," it means that they own it or have control over it. The opposite of this phrase could be "in one's own possession" or "under one's control." Other antonyms for this phrase may include "lost," "stolen," or "misplaced." If something is lost, it means that it cannot be found, and no one physically possesses it. If something is stolen, it means that someone has taken it without permission or legal ownership. Lastly, if something is misplaced, it means that it is in a location that is not where it is supposed to be, but it is not necessarily lost or stolen.