aop_case_events, field case_id empty (not null)

I know I have logic hooks implemented for the inbound mails creating cases.

How come the aop_case_events having a filed case_id which does not accept null have blank case_id?
Should I care about aop_case_events when I update a case after_save?

Even much bizarre aop is not active in my configuration… but aop_case_events is populated…

Serge