She would have had to be very sensitive to catch the nearly imperceptible flinch of the vehicle when she made contact. He certainly wasn't going to break cover because an organic was inquisitive, but he wasn't going to pretend to enjoy them touching his armor and getting their grim all over his plating.

"You are interrupting an official surveillance operation. Please return to your business."
An official Autobot operations at the very least. Magnus was banking on his alt mode's possible military association to instill some respect. Hopefully the creature wouldn't call his bluff.