As I read my dear friend Dolores' blog at http://movingforwardwithalzheimers.blogspot.com/ this morning I knew that I had already walked that path and perhaps some suggestions I found helpful might be of benefit to her and to the rest of you.
David has begun to wander. Oh, I remember those days. Scary, yes. You just never know when they will become confused about their space or not know where you are... or where 'home' is, and head out the door. Elbert was forever trying to go 'home' so I tried the chain on the door.
I found great peace of mind in a very simple solution. My son in law screwed in very small door alarms on all my doors and I knew everytime Elbert opened an outside door. These alarms can be found at Lowe's or Home Depot (maybe even Walmart, I haven't looked there). They come in two small pieces, one to attach to the edge of your door, the other goes on the door frame and when the door is opened the alarm goes off.
Hope this helps.
Peace of mind is a wonderful thing.