no, the firmware does any maintenance. good firmware should do gradual scrub whenever it's idle. unfortunately, there's no real way to know whether the firmware is good, or doing anything.
I wonder if there's some easy way to measure power consumed by a device - to detect whether it's doing housekeeping.
Honestly this is one of my favorite things about ZFS. I know that a disk scan is performed every week/month (whatever schedule). And I also know that it has verified the contents of each block. It is very reassuring in that way.
You've validated that the scrub is actually running, right? I know that the lack of a default schedule for ZFS scrubs caused Linus Media Group to lose a bunch of archived videos to bitrot.