I have searched and thought about it but cant figure out how/if one can monitor a sliding glass door latch to determine if it is in the locked (DOWN) or unlocked (UP) position. maybe a very small mercury switch? I have a door sensor to monitor it is indeed closed, but I tend to forget to lock it
I’ve thought about this too. Imagined a sensor in the door itself, attached to whatever rod moves when the latch is opened/closed. But then it would be in a metal door, which would likely pose transmission issues and be difficult to change batteries etc. Same issues if the thing is in the door frame.
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Can you post a picture of the door with the surrounding wall and door frame? There are some possibilities, but it depends very much on the exact architecture of the door.
The first issue, and this is important, is it generally not a good idea to post a surface mount magnet piece below about 3 1/2 feet off the floor. That’s because if either a child or a dog manage to get the magnet piece off the door and swallow it it’s a very serious medical issue. 
There might also be issues with the metal frame, but if it’s a patio door, these are more commonly aluminum, which doesn’t present issues. But if it’s a steel frame, that is a problem. (You can tell whether the doorframe is ferrous just by taking a regular kitchen magnet and seeing if it sticks to it. If it sticks, then there are going to be issues with using magnetic sensors.)
So the best approach is to use a recessed sensor or microswitch in the wall such that when the latch closes it makes contact. A couple of community members have done this for some exterior doors, but I don’t think anyone’s done it for a sliding door.
Another alternative is to put the micro switch inside the turn bolt, but I think that most of the ones for patio doors would be too small. But again it depends on the specific door you are working with.
So it all depends on the wall around the door frame as to whether that’s doable or not. And on the material of the door frame itself.
Here are some project reports that present the general idea:
Here’s one with the switch inside the turn bolt casing. Note that that’s a very old thread and discusses the “thingshield” which is no longer made. But there are other options available now (2019) which would accomplish the same thing.
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To this day I still have that device (my last one too), but it will soon be replaced by a Kwikset Convert. That little microswitch has held up very well, and I just use the “Z-Wave Door/Window Sensor” default DTH.
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You could install a microswitch on the jamb side inside where the hookbolt comes up and in, if there’s room. Then you could run small wires from there, under the molding and right into the ecolink sensor. If you place the sensor right up next to the molding you can easily hide the wires and it will look pretty clean. Sounds like a fun little project to dive in to!
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