Her guess is right, because Skulduggery is mad! He's grumpy and annoyed, to boot! But it's impossible to tell between the glare, the tint, and the skull. Maybe it's in the way he hits the steering wheel a few times as Max glides past the cactus in question...

Alright, fine. He honks a few times in resignation and defeat. And then one long honk to stand in for the censor over all of his extensive swearing.