ZFS HDDs suddenly wake up from stand-by

I have FreeBSD 10.2 release installed on a server which I sometimes use as a NAS. For this purpose I have a separate ZFS pool on separate disks. I bring them online only if I need them to save power. On the other side I have written a small script which automatically export the ZFS pool and send the corresponding disks into stand-by if I don't need them. The problem is that although the disks are sleeping they come back online from time to time.

Does someone know how this can happen? No system service uses files from this pool. Does FreeBSD offer tools to detect commands which are sent to the NAS disks to figure out the reason?