It looks like after stepdown of the config server primary, if the dist lock manager had any locks which it held (or failed to unlock) while it was a primary, it will keep trying to unlock them even though it is not a primary anymore. This pollutes the logs with these messages:

2018-07-25T13:32:29.635-0400 W SHARDING [replSetDistLockPinger] Failed to unlock lock with ts: 5b45d865f11548390b954ea2 and _id: config-movePrimary :: caused by :: NotMaster: Not primary while running findAndModify command on collection config.locks