Plenty of “secret” projects that are not actually secret for whatever reason and require no one breaching US national security regulations. Beyond that, without people even needing to say specifically what they working on, how it works, etc — you can tell just by the way for example Demis Hassabis and Ilya Sutskever both talk that they’re both hyper focused on finding one of the many paths to AGIs.

Also, agree that if such a monitoring system did exist back then was likely combination network velocity and honey pots - and if need, tracing traffic to identify and breach the originating system to understand the source of the behavior.