The Ferrari's plate was changed to Higgins-2, but when they get the car from the impound it has the plate Robin-2 again without explanation.
At the end when Higgins and Magnum go to the impound lot to get the Ferrari, Magnum has a gas can with him to add gas to the car, as the reason it was there is because he ran out of gas on the road. As he walks up to the car the gas cap on the car is already open so that he can immediately start pouring gas into the gas tank. But there is no reason for it to be open already, since this was the first time he had been to the car since he had to abandon it, and no one else would have had a reason to open the gas cap (or even be able to open it) before that moment.
When Magnum and Higgins break into Frank's house, they are faced with an alarm system counting down. Higgins defeats it by switching off the circuit breaker. Alarm systems all have batteries in them to prevent exactly that. Given Higgins background she would know that.