Too long to summarize here, Sanderson methodically asked, not what UAOs were, but what they could be. He developed a six page outline of the possibilities. Thus, they could be inanimate or animate. If inanimate, they might be natural, or artificial, each possibility with several subsets. If animate, they could also be natural or artificial. Natural forms might include life-forms indigenous to space, or to atmospheres, or to solid bodies. Artificial forms might be domesticated natural life-forms, genetically created life forms, or biochemically created life forms. And so on.
Sanderson at several points suggested the possibility that the "occupants" of UFOs might be artificial life forms. He was not dogmatic about this, and also entertained the idea that they might be an as-yet unknown life-form indigenous to Earth. Still, the concept of UFOs as a form of artificial intelligence is fertile enough that we might have expected some follow-up.
After all, it’s been 35 years.
Documentary with background information on the USO research of Ivan Sanderson and his work with the British Intelligence Services - also a good interview with Bill Chalker about a Water/UFO case in Australia: