If a cat marked that and left solids that heavy, it would have stained the grille cloth, too. Cats tend to spray corners, edges, and objects that they can "get their legs over." These speakers don't fit that description. Plus that cat's got some awfully thick fluid there. Not saying it's impossible, but seems improbable.
OTOH, I'm wondering if the speakers ever went from a cold to a warm, humid environment at any time. If so, some rapid condensation on the cold metal, collecting and running down a dusty driver could produce that pattern.
As an alternative, someone could've just spilled some water or other liquid and it spattered through the grille cloth and hit the metal cap, disturbing and redistributing a fine coating of dust there. The cloth dried out, but the spill pattern remained on the dome.