What does the gold coloured symbol that looks like a wind up / radiator bleed valve key represent?

Those are the golden wind-up keys for the Detectives Needed Challenge (DNC). When the event is active, you can tap on the stopwatch icon to see how many keys you need, the rewards, your progress, and whether keys are earned from playing scenes and/or completing Help Wanted tasks.

There are also keys that you earn to open more scenes in the Secrets side story. It is more bronze in color and has 3 loops on it. You earn them only by playing regular scenes and can only hold a max of 500 bronze keys at a time.