Draining mode unlocks all of the config server held distributed locks. The Balancer reacquires the {_id: "balancer"} distlock after drain mode has been left.

Theoretically, a 3.2 Mongos could take the {_id: "balancer"} distlock in this window. It could briefly hold the distlock for a manual moveChunk, or indefinitely if auto-balancing is enabled. It could be held indefinitely if the config server is holding collection distlocks for migrations it is trying to recover after failover.