What is the source of syslog message "Condition check resulted in MD array scrubbing"

It simply means that the service responsible to continue a scrub was skipped because it was not necessary - ie: the condition to run it was not satisfied, for example because not much time passed since the last scrub.

More specifically, the unit mdcheck_continue.service check for the condition ConditionPathExistsGlob = /var/lib/mdcheck/MD_UUID_* meaning that it check for started-but-interrupted scrubs. You see the message each day because a timer called mdcheck_continue.timer is configured to start, each day, the mdcheck_continue.service unit.

So:

• mdcheck_continue.timer elapses;
• mdcheck_continue.service is invoked;
• as no array should be scrubbed, mdcheck_continue.service logs and exits.