I am dealing with a Flash file which has been created by a third party. It has a lot of folders and files in the library and I can\'t seem to find any ActionScript anywhere.
In addition to @grapefrukt's answer: Make sure you toggle the ShowActionscript button in the Movie Explorer. That would be the third icon, the one with the blue arrow.

You can use the Find field to look search for common actions like: stop(), .visible, etc.
Alternatively you can use the Find And Replace Panel (Ctrl+F / CMD + F) and search for Text (common actions) and tick ActionScript only.

Note: The Find And Replace Panel is a separate panel, do not confuse it with the Find And Replace functionality in the Actions Panel.
Also, I think you could find actions using the Script Navigator, inside the Actions Panel's Toolbox:

HTH