Not really a side-channel, but remember stuxnet? Airgapped networks are rarely truly isolated, you still have to get data in and out every now and then, in principle after careful vetting. But the AI could manipulate the files that are carried out for example.

And those AI companies also do robotics research, and this is entirely speculation but it'd be on-brand to also have AI watching security cameras, so some blinkenlights communication between AIs may seem like a movie plot, but so does a swarm of AIs collaborating to break out in the first place...