Sliding glass doors are secured by latches not locks. They are vulnerable to being forced open from the outside because of these inherently defective latch mechanisms. This can be easily be prevented by inserting a wooden dowel or stick into the track thus preventing or limiting movement.
Patio bolts are a must as a secondary backup lock for sliding doors. Correctly positioned they prevent movement of the sliding glass door. On the down side sliding glass doors cannot be unlocked using the primary lock from the outside whilst the patio bolt is engaged from the inside, as patio bolts are usually fitted to the inside of the sliding glass door frame.
Older sliding glass doors can be lifted up and off their track and thereby defeat the latch mechanism. To prevent lifting, you need to keep the door rollers in good condition and properly adjusted. You can also install anti-lift devices such as a pin that extends through both the sliding and fixed portion of the door.
There are also numerous locking and blocking devices available in any good quality hardware store that will prevent a sliding door from being lifted or forced horizontally. Place highly visible decals on the glass door near the latch mechanism that indicates that an alarm system, a dog, or block watch/operation identification is in place. Burglars dislike alarm systems and definitely big barking dogs.