This message is logged by scrub_lease() in server/failover.c:

 void scrub_lease(struct lease* lease, const char *file, int line) {
     log_debug ("%s(%d):scrubbing lease for %s, hostname: %s", file,
 line,

--

It seems to be the only log_debug statement in the failover code that is not surrounded by a "#if defined (DEBUG_FAILOVER_MESSAGES)" statement, which seems to be an oversight.

The ode that introduced this logging stanza was added to ISC DHCP with this change:

- Leases are now scrubbed of certain prior use information when pool
  re-balancing reassigns them from one FO peer to the other.  This
  corrects an issue where leases that were offered but not used
  by the client retained the client hostname from the original
  client. Thanks to Pavel Polacek, Jan Evangelista Purkyne University
  for reporting the issue.
  [ISC-Bugs #42008]