We rented a car for 2 weeks in Sicily twice, once in 2015 and again a year later. It was relatively uneventful. If there were ZTLs, I did not see them. On an autostrada, we saw a car tailgating a police vehicle, and both were going at least 20 kph over the speed limit. Apple Maps offered us an opportunity to drive the car down a flight of stairs, but we declined. We never drove at all in or near Palermo.
We received no fines, no tickets of any kind.
These experiences turned out to have poorly prepared us for driving in northern Italy in 2017. We picked up a rental car in La Spezia, and had it for 12 days. 4 months after returning, I got the first bill , about €25, from Hertz for furnishing the Italian authorities our information re a ZTL violation in Prato, about 2 hours after we rented the car. Gulp. I thought I watched carefully for ZTLs. All we did was drop a friend at the train station and leave town.
From reading these forums, I expect, sometime in the next few months, to receive from those authorities a fine, and instructions how to pay it.
What will interfere with my sleep is wondering what else I unwittingly did during the ensuing 11 days. We never went near Rome in the car, and in Orvieto & Gubbio we parked where we were told to by our hosts, and never drove around in town. I'm worried about Montepulciano, though. Time will tell.
Lessons learned driving in Sicily, such as it being OK to tailgate a police vehicle while speeding on an autostrada may not be applicable to other locations in Italy.