The postmaster is not a storage system - it's a safety net for things you missed in mission. It only holds 20 items. Once that limit is reached, using the FIFO method, new things push old things out.
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It is possible that it happened, is it possible to recover it somehow?
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Sorry - no. Bungie will not replace lost or stolen items, and it is the player's responsibility to make sure they are cleaning out the postmaster on a regular basis - or better yet, picking up items in the game as you play.