Looks like there was an user 'Config' with priv level 15 who came in and worked on your ASA. Time to start asking questions around your peer group.
Check all other user group which has priv lvl set to 15. Maybe, some user came in, created this user 'Config' on the fly and did whatever that user did. And then it deleted that user 'Config' and removed log files as well while being at it.
Would be a good idea to lower all other user levels to 7 atleast for the moment, till your figure out who it was. And yes remember to keep atleast one admin level user so that in the future atleast you can log into global config mode.
See how it goes, and keep us posted.
Picking pebbles on the shore of the networking ocean