Re: "...can pick up the signal from keys..."
"Keyless Car Starting is probably using RFID. That's much shorter range. The hackers need to get within a meter or two of your keys, so their can 'illuminate' it with enough RF to power it up. The system designers should include some handshaking, not just an easily copied serial number."
Thieves use two way repeaters to steal cars. Increasing the size of the antenna increases the range of RFID devices.
Some guy used a hacked Vtech toy with a large antenna to read RFID tags from 10m away.