(I moved this to projects so you can get individualized answers based on your specific set up.)
The short answer is that this can almost always be solved with multiple motion sensors assigned to a zone and positioned in appropriate places. And masked, where necessary, to narrow the size of the detection zone.
For an L-shaped stairwell typically a combination of a ceiling mounted motion sensor and some motion sensors on individual stair tread’s with narrowed detection fields will do it.
@mike_maxwell , for example, has used a small tube over the lens of the motion sensor to narrow the field significantly. Lots of other people have just used masking tape in various patterns.
You might also look at the project reports on the quick browse lists in the community – created wiki. If you look near the very end in the room section, there is a list for “hallways and staircases” that might give you some ideas.
You can go to some other kind of sensor, But it will probably cost more and be more complex to integrate with SmartThings.
Here’s the code for creating a zone:
And a little more on ceiling-mounted motion sensors:
