Continuing your search you find a number of natural cracks. Further evidence that this passage is natural and not dwarven in nature. You do however find a section of the low roof when has been cut smooth, probably to stop tall humans banging their head at that point, so the dwarves while they did not dig the passage have been present sometime in the distant past.
Other than all this, your searches for hidden objects, passages, doors and door releases, etc., comes to naught. Most disappointing you feel.