There is an option under Office->Overview to see flights without service/handling contract (if an external or internal service/handling contract exists).

I tried to disable a handling contract for a particular flight number, to see if it appears in this section under Office->Overview->Flights without service contract, but it did not.... what am I doing wrong?
Make sure you have the check-marks at the bottom right checked:
Settings
You can check another office to see if they appear or not :)
The key thing is making sure the checkboxes on your office overview page are ticked.

I’ve always disliked that the option isn’t enabled by default when opening an office. I can’t reason why it wouldn’t be, since you’d have to go through the process of signing a contract or allocating capacity before anything appears in the report area anyhow.