Peter Brook states that ‘I can take any empty space and call it a bare stage. A man walks across this empty space whilst someone else is watching him, and this is all is for an act of theatre to be engaged.’
So yes, according to one of the most respected directors of the last 50 years, if something is observed it is theatre. Beckett did write a play with no actors, but he was being facetious.
I suspect further discussion of the nature of theatre is a bit out with the scope of the thread, but I will say Alan Cumming’s one-man Macbeth (c. 2012) was one of the best things I ever saw. Apologies for the tangent!