Modern car and vehicles gas tanks have an anti rollover valve in the openings of the gas tank to prevent fuel spillage in the event of a rollover accident.
So those valves make it harder to siphon gasoline out of the modern car but the valves don't completely stop gas theft and gasoline can still be siphoned from a modern car.
A smaller diameter hose can fit in around the valve and allow a person to siphon gasoline from the tanks.
Cars with no gas caps make it even easier for someone to siphon gas because there's now way to install a locking gas cap which is a bad thing.
The way I prevent gas theft from my car is I have a locking gas cap on the fuel filler hole which prevents the gas cap from being taken off without a key and I also added an additional lock to the gas door.
This prevents the gas thieves from easily accessing the gasoline in my car and I used to have a problem with people stealing my gasoline from my car even when at work etc and after adding the lock gas cap and locking gas filler door and no longer had my gasoline stolen.
The gas can still be stolen by drilling holes etc underneath the car but adding the locking gas cap and lock to the gas door helps prevent the gas theft because the thieves would rather take the easy way of getting the gas.
I also lock my vehicle in my locked garage overnight or when I'm not driving it as they also helps keep the car from being an easy target for thieves.
If you have no garage then add the locking gas cap and lock to the gas door and install cameras to help catch the gas thieves in action.