Issue remains: the mac@ snap; and recovery, generated by Hardware Growl which I like..
As a result of this thread I turned Hardware Growler and Growl back on and since then I have seen occasional Notifications of something like
Macsnap:xxxx mounting and almost immediately dismounting. while I haven't accurately captured the notification message it
appears to be mounting/creating a
snapshot of the APFS volume. Could this explain your mac@snap?