It's not standard headline grammar by definition. They do leave out function words (A mouse was), but in the same space, you could write: "Mouse tidying a man's

It's not standard headline grammar by definition. They do leave out function words (A mouse was), but in the same space, you could write: "Mouse tidying a man's shed every night secretly filmed." In which case "secretly" stands out as superfluous, because nobody expects you to inform the mouse beforehand.