Hi there,
This is not what spawns Tip Off messages... I've not previously given away the secrets so I'm tempted to let it stay a mystery for now...
As for the timer, I don't remember Dav speaking about a the non-station missions, but I did in my Q&A just before christmas.
Staying in a game area, undocked, in a populated system, for 15 minutes will trigger a call for a mission. You won't necessarily get one, nor will you necessarily know you've got one. The request will go out, though.
Most of these missions are hidden to start with and just subtly manipulate the game world around you by spawning ships or USS events. Once you've performed an pertinent action, you get an inbox message with further details.
Hope this helps,
Dom