Other mistake: When Roy's talking to Dixie while John's waiting out in the squad, a call comes over the HT that Roy's holding, and Squad 51 is asked to respond to a 'possible heart case'. Roy pulls a piece of paper out of his pocket, and as we hear the dispatcher giving the address, we can see Keven Tighe softly mouthing the address at the same time that it's being given for the first time.
Other mistake: When the boy is brought to the ER by his babysitter, they hook him up to the ventilator and we see the bag dramatically showing inflation and deflation in the closeup, but in the wide shots the bag is deflated and doesn't move.
Answer: Yes, there's a thin cable secured to the sign's bottom mounting bracket which leads upward to the crane arm, and can be seen in quite a few shots at various angles. It's reasonable to presume that this cable which is secured to the crane was part of the sign's mounting process to the two wall brackets before the winds caused the accident, the premise of this rescue. For this reason it really can't be noted as "stunt wires being visible" or something like that.
Super Grover