The message you are referring to appears when an external entity deletes a topic for which you have created an alert.
So, in this case, there was an alert set for the topic and someone deleted the topic outside of Lenses.
The lenses-system is the software itself. It keeps a table with all the resources it needs to track. During the periodic check of the alert rules, it discovered that a tracked topic no longer existed and updated its database.
If the topic had been deleted from within Lenses, then the message would be Topic removed instead of Dataset removed .