There are things other than Cars and Factories that pump out CO2. Mass destruction of forests for example. A cataclysmic event that causes forests to wither and die. The decomposition of the foliage would produce a spike in CO2 levels. Nothing like todays culprits, granted, but, in terms of Geological time, the effects would appear similar.
5 Million years from now, Archaeologists could dig up STone Axes, Dug-out Canoes and a Nuclear Reactor, and deduce that they were all manufacted at "around the same time". It depends on one's perspective.