I think at that point you should use a public module such as
Which features events for when a player enters and leaves a zone
If you still wanna do it yourself
Have a table set up for old players that were detected
Every time you check the region3, check the player sthat were found in it via another table and then compare with t he old table if it exists, if any of the players recently found were in the old table but not anymore, then they left
It’s a bit confusing for Region3s, although my method is probably not the best