TRS session question

[Approach_Medium]

Hi;
I'm curious.
When I'm in TRS2006 Surveyor, working without a session open, I place a simple sign post (sign-1) and give it a name, so I can locate a specific area later.
When I save the map, TRS creates a session file as well.
Why does it want to create a session when I have not run any trains, or even placed any on the map?

Thanks

FW