Well, they may not be 'leaked' as, we keep trying:
2017-06-06 21:16:52 ERROR juju.state unit.go:339 cannot delete history for unit "u#ntp/186#charm": <nil>
2017-06-06 21:18:35 ERROR juju.state unit.go:339 cannot delete history for unit "u#ntp/186#charm": <nil>
2017-06-06 21:26:54 ERROR juju.state unit.go:339 cannot delete history for unit "u#ntp/186#charm": <nil>
2017-06-06 21:33:07 ERROR juju.state unit.go:339 cannot delete history for unit "u#ntp/186#charm": <nil>
2017-06-06 21:34:27 ERROR juju.state unit.go:339 cannot delete history for unit "u#ntp/186#charm": <nil>
2017-06-06 21:35:29 ERROR juju.state unit.go:339 cannot delete history for unit "u#ntp/186#charm": <nil>
2017-06-06 21:36:51 ERROR juju.state unit.go:339 cannot delete history for unit "u#ntp/186#charm": <nil>
2017-06-06 21:37:54 ERROR juju.state unit.go:339 cannot delete history for unit "u#ntp/186#charm": <nil>
2017-06-06 21:39:11 ERROR juju.state unit.go:339 cannot delete history for unit "u#ntp/186#charm": <nil>
2017-06-06 21:40:26 ERROR juju.state unit.go:339 cannot delete history for unit "u#ntp/186#charm": <nil>
2017-06-06 21:42:11 ERROR juju.state unit.go:339 cannot delete history for unit "u#ntp/186#charm": <nil>
2017-06-06 21:43:14 ERROR juju.state unit.go:339 cannot delete history for unit "u#ntp/186#charm": <nil>
2017-06-06 21:44:24 ERROR juju.state unit.go:339 cannot delete history for unit "u#ntp/186#charm": <nil>
2017-06-06 21:46:01 ERROR juju.state unit.go:339 cannot delete history for unit "u#ntp/186#charm": <nil>
2017-06-06 21:49:32 ERROR juju.state unit.go:339 cannot delete history for unit "u#ntp/186#charm": <nil>
2017-06-06 21:50:46 ERROR juju.state unit.go:339 cannot delete history for unit "u#ntp/186#charm": <nil>
2017-06-06 21:52:21 ERROR juju.state unit.go:339 cannot delete history for unit "u#ntp/186#charm": <nil>
2017-06-06 21:54:19 ERROR juju.state unit.go:339 cannot delete history for unit "u#ntp/186#charm": <nil>
2017-06-06 21:55:39 ERROR juju.state unit.go:339 cannot delete history for unit "u#ntp/186#charm": <nil>
2017-06-06 21:57:03 ERROR juju.state unit.go:339 cannot delete history for unit "u#ntp/186#charm": <nil>
2017-06-06 21:58:23 ERROR juju.state unit.go:339 cannot delete history for unit "u#ntp/186#charm": <nil>
2017-06-06 21:59:49 ERROR juju.state unit.go:339 cannot delete history for unit "u#ntp/186#charm": <nil>
2017-06-06 22:01:27 ERROR juju.state unit.go:339 cannot delete history for unit "u#ntp/186#charm": <nil>
2017-06-06 22:03:16 ERROR juju.state unit.go:339 cannot delete history for unit "u#ntp/186#charm": <nil>
2017-06-06 22:04:33 ERROR juju.state unit.go:339 cannot delete history for unit "u#ntp/186#charm": <nil>
2017-06-06 22:05:48 ERROR juju.state unit.go:339 cannot delete history for unit "u#ntp/186#charm": <nil>
2017-06-06 22:07:12 ERROR juju.state unit.go:339 cannot delete history for unit "u#ntp/186#charm": <nil>
2017-06-07 14:48:01 ERROR juju.state unit.go:339 cannot delete history for unit "u#ntp/186#charm": <nil>
2017-06-07 14:57:15 ERROR juju.state unit.go:339 cannot delete history for unit "u#ntp/186#charm": <nil>
2017-06-07 14:59:27 ERROR juju.state unit.go:339 cannot delete history for unit "u#ntp/186#charm": <nil>
2017-06-07 15:00:55 ERROR juju.state unit.go:339 cannot delete history for unit "u#ntp/186#charm": <nil>
2017-06-07 15:02:58 ERROR juju.state unit.go:339 cannot delete history for unit "u#ntp/186#charm": <nil>
2017-06-07 15:04:50 ERROR juju.state unit.go:339 cannot delete history for unit "u#ntp/186#charm": <nil>
2017-06-07 15:05:59 ERROR juju.state unit.go:339 cannot delete history for unit "u#ntp/186#charm": <nil>
2017-06-07 15:12:38 ERROR juju.state unit.go:339 cannot delete history for unit "u#ntp/186#charm": <nil>
2017-06-07 15:14:55 ERROR juju.state unit.go:339 cannot delete history for unit "u#ntp/186#charm": <nil>
2017-06-07 15:16:01 ERROR juju.state unit.go:339 cannot delete history for unit "u#ntp/186#charm": <nil>
2017-06-07 15:17:26 ERROR juju.state unit.go:339 cannot delete history for unit "u#ntp/186#charm": <nil>
2017-06-07 15:18:36 ERROR juju.state unit.go:339 cannot delete history for unit "u#ntp/186#charm": <nil>
2017-06-07 15:19:50 ERROR juju.state unit.go:339 cannot delete history for unit "u#ntp/186#charm": <nil>
2017-06-07 15:21:06 ERROR juju.state unit.go:339 cannot delete history for unit "u#ntp/186#charm": <nil>
2017-06-07 15:25:03 ERROR juju.state unit.go:339 cannot delete history for unit "u#ntp/186#charm": <nil>
2017-06-07 15:26:38 ERROR juju.state unit.go:339 cannot delete history for unit "u#ntp/186#charm": <nil>
2017-06-07 15:29:25 ERROR juju.state unit.go:339 cannot delete history for unit "u#ntp/186#charm": <nil>
2017-06-07 15:30:53 ERROR juju.state unit.go:339 cannot delete history for unit "u#ntp/186#charm": <nil>
2017-06-07 15:32:52 ERROR juju.state unit.go:339 cannot delete history for unit "u#ntp/186#charm": <nil>
2017-06-07 15:35:26 ERROR juju.state unit.go:339 cannot delete history for unit "u#ntp/186#charm": <nil>
2017-06-07 15:53:39 ERROR juju.state unit.go:339 cannot delete history for unit "u#ntp/186#charm": <nil>
2017-06-07 16:06:26 ERROR juju.state unit.go:339 cannot delete history for unit "u#ntp/186#charm": <nil>
2017-06-07 16:08:41 ERROR juju.state unit.go:339 cannot delete history for unit "u#ntp/186#charm": <nil>
2017-06-07 16:11:57 ERROR juju.state unit.go:339 cannot delete history for unit "u#ntp/186#charm": <nil>
2017-06-07 16:18:25 ERROR juju.state unit.go:339 cannot delete history for unit "u#ntp/186#charm": <nil>