It is sad in its docs:
The btGhostObject can keep track of all objects that are overlapping By default, this overlap is based on the AABB This is useful for creating a character controller, collision sensors/triggers, explosions etc
Does it mean it is only AABB of its underlying mesh and so detects only collisions with AABB and not complex mesh structure?
There is no difference for collision dispatcher between a ghost object and a rigid body: they behave the same way both in broad- and narrow-phase. If you iterate over dispatcher's manifold array, you'll see proper collisions with ghost objects. Documentation refers to the btGhostObject internal overlapping objects cache accessible via getOverlappingObject(int index) / getNumOverlappingObjects() methods. This cache is populated in the broadphase (thus obviously using only AABB), and you have to set the proper pair callback for this to work (world->getPairCache()->setInternalGhostPairCallback(new btGhostPairCallback);). You can iterate over this cache in the post-tick and check for the actual collisions.
