Yes.They just need a scanner to get the frequency by scanning near where your key is (usually near the main door) and drive off your car within 2 minutes. These scanners are really easy to buy these days.
However this is only easy to do when there is no encryption at both end. Its simple to apply.
A basic keyless system with no encryption is very easy to clone. Just imagine it like the rfid card we use for some apartment access or elite housing area access or factory. Most of these system does not have encryption thus u can copy the card whenever u need or lost or have ill intention as the system only recognize and need the badge id to work. Some manufacturer tied the programming to work with their software only but wont offer the best protection as sometimes other device such as rfid guard tour tool can still read the badge id. Again, all u need to do is replicate the badge id.
The one equip with encryption is more secured. Its not foolproof but it is very hard to decrypt unless u have the decryption key.
Now there is already tech that have decryption and 2 way comm for a very secured card access. I know card access and car alarm sound different but trust me their basic working is the same. How this one work is by sending an encrypted signal everytime we lock and unlock the thing and at the same time set a future encrypted code for the next transaction. This code is as a code for the alarm systen to only recognize signals from the original device.
This way, bad guys wont know the 'username' and the 'password' as its kept on revolving. To make things worst for them, its encrypted.
Which keyless alarm uses this tech?
Dunno, and its best the thief dont know and the manufacturer dont declare.