MongoDB 3.2.1 3-node RS
Had a secondary out of sync for some time (a few weeks). Maintenance mode count was in thousands, if it matters.
Shutdown command was issued on the secondary 4 days ago, but it was still running. After killing it with kill-9 today and starting it back up, it went into full resync. This points out to a possible problem with WT data consistency after kill-9.