Direct physical access by the attacker isn't strictly necessary (i.e. operation Olympic Games) to "network" a computer you otherwise believe isn't networked. Unless you're bootstrapping from nothing attackers have tons of potential "ins" (firmware, the operating system, application software) to introduce backdoors or side-channels.

I've very nearly reached the point of just assuming all "modern" computers are effectively "networked", even if only by ultra-low bandwidth, exceedingly high-latency unidirectional side channels. Just bringing an "untrusted" computer into proximity of a "trusted" computer (say, having a smartphone in your pocket) might be enough to allow for exfiltration of data from the "trusted" system (assuming there's a side-channel in the "trusted" computer you're unaware of).