I once had an annoying cat (hey, I'm a dog person) that pushed in the dust cap of a woofer with her paw, leaving an indentation like you described. I don't really suggest you try this but at the time, I used a piece of sticky tape on a stick, just touching it hard enough to the indent so that it adhered, then I pulled it out and the indentation popped out.
But if you try it, you might accidentally puncture the dust cap, so don't worry about it. In my case, I was annoyed at the cat and the way the dust cap looked.
Axiom Resident Expert