Nothing really, if they are so determined they are willing to break into your garage. Most career criminals know how to get around alarms of most automotive and home types.
True but what alarms really offer is
time, time is the true defender of ones possessions.
No alarm, sad sack owner - gone in a second. Alarm whaling and things chirping, makes the burglar rush, get nervous and in most cases leave the scene. People often think alarms are there to stop car theft when although this "true" and how they are sold to the public, it's not "really" the case, they're only there to delay the process and warn anyone within the immediate area that the car is being broken into.
Any alarm system can be broken in to (almost all) but to do most the buglar would basically have to take the car back to his garage for 10 mins, 20, mins 30 mins+ to get past some of these systems, the good systems anyhow. The cheap local car alarms you get with a free balloon are utter shlt, spend good money on the best there is and there
is something you can do about it. With or without is up to the owner, I'll take any precaution I see fit to save my pride & joy(s).