This is interesting but may just be a settings issue. I’m curious how the task was moved without detection? Was it unassigned from the original owner, then reassigned to the person who had it in their My Tasks list? Was it copied to another project then removed from the original? Several possible scenarios!
Depending on your notification settings you and/or project/task owner, should at least receive an email when a task is moved, reassigned, etc. You would have to be following the task as a collaborator if you did not originally create/assign it.
If you have the Business version of Asana, you can run reports to show tasks in various stages (ie: complete, incomplete, overdue, etc.) to locate any tasks that may be missing.
Or, in the main search box type “tasks”. This will return a list of every task across all projects, should this happen again