Yep, it's usually someone with a router that doesn't have the ports forwarded correctly.
FS2002:
TCP 47624
UDP 2300 to 2400
FS2004 and FSX:
UDP 23456 (or your "2004 host port" on the Server / Options / Session tab)
UDP 6073
UDP 2302 to 2400 (not necessary if also forwarding 2300 to 2400 for FS2002)
The "Not in plane" message just means that FSHost isn't receiving any location data for the player, which usually means they're at the FS startup screen and haven't started a flight yet. But it can also be because of port problems as you said, especially when the player is having trouble getting into the game.
Russell