I think all your questions are covered in the manual. They are also cover in the tutorial at
http://youtu.be/31GcDjw1AQs.
If so, what light is rendering at that click?
But default, a new Octane scene will have a gray texture environment, which would be lighting your scene.
If I look into the plugin, under the rendertarget setting, it shows "environment = texture environment" and the sub-node says "texture = bw-pool.png". This is the LDR image from the IBL light. I can see it rendering as the backdrop of the test render if I render without any physical background. I guess this IBL is rendering as a backdrop and not a light?
Yes, no physical background is needed with Octane texture environments. You can add sunlight or other light sources in addition to the IBL with emitters.
If I delete all of the lights from the scene, refresh the scene in Octane, then render again, the backdrop is gone. But it still renders brightly. So where is the lighting coming from?
You deleted the IBL light, so the texture environment is replaced with a flat gray color so you can see something in the render. Set the power to 0.001 if you don't want that light in the scene.
If I switch in the "environment" dropdown from "texture environment" to "daylight" and then back, the bw-pool.png is gone. And then again, I have no idea what light set is rendering the scene, since the nodes do not list lights. I'm guessing the scene is lit from the "Environment" node?
That is correct - that is the same behavior that Octane Standalone has. There are no "lights" in the environment - only sunlight, or an IBL texturemap. Any emitter lights will be in the Materials Tab (since an emitter is a material function, not an environment function).
Also, is it possible to offer a .tif export option so we can export the image with alpha? I also usually render in PP2012 and export an additional z-depth pass. How can this be achieved in Octane with the plugin? I'm guessing it's not something that is possible at this time?
This is covered in the manual, but in summary it's a kernel setting.
Hope that helps.
Paul