Are their any firm rules for when to separate a participle phrase with a comma. I rarely place a participle phrases close to the noun it modifies and wonder whether a comma is required.

Do either of these sentences require a comma?

In their dressing gowns, Rose and Martin stand by the window (,) watching the scene unchanged from the night before: the storm continues.

Rose and Martin watch the storm, not the window, but I think that would be understood.

They view the top of the cliffs where scores of youths are positioned (,) using the advantage of height for an aerial bombardment of rocks.

Here there is no object or indirect object, so I think the comma is unnecessary.