Annoyed someone called the police when he vanished? What a schmuck...
The symptoms displayed by Mark here are very similar to some kind of "classic" pre-suicidal actions (i.e. "putting his affairs in order") so I can definitely see why suicide would be a concern among his friends.
It's not a huge stretch to follow that train of thought to the notion that his actions amounted to a somewhat obscure suicide note. And giving away your stuff/deleting accounts en masse/shutting your life down is a fairly classic behavior among suicidal individuals.