Certainly not for neutral ones, which is what aliens generally start out as. Hostile ones have the flags set for new contacts and updates, but does updates include reacquisition?
I'm pretty sure I've not been getting events when something reappears on sensors. That's one of the reasons why the message logs were are so empty when looking for the cause of an interrupt loop.
I've got problems with mechanics like ground combat, sto and missile defense being tied to hostile diplo status anyway (let them be independent of that), because that way you can't decide to only start a fight on one body, or ignore certain neutral empires in shared systems (Sol), but react to them entering your claimed space.