Most sliding doors can be retrofitted with a lock cylinder. If your sliding door doesn’t have a lock on it and you want to be able to enter through this door, check to see if there is a small circular indentation in the middle of the handle on the outside. If there is, this is a door that can be retrofitted. The round metal perforation can be knocked out and, when the handle is disassembled, a specially shaped knob cylinder can be added. You can come and go from your sliding door as you please. You can also add a sort of deadbolt to your sliding glass door that cannot be jimmied with a hanger either.
Leaving your sliding door slightly ajar is very appealing during the hot summer months. The only problem is keeping two-legged skunks from getting in! They will use long pieces of metal to reach around the door to move a dowel or other item from the door slide and enter. You can keep them from doing this with a drop bolt. They are inexpensive to procure and install. A cylinder is inexpensive to put in your door as well. I can put a cylinder in your sliding door and a drop rod on it as well for about $150, keyed to your other locks. Give me a call if you want me to do this for you. I can usually get to a job within half an hour of a call.