Hoping to get help and/or pique interest. I remember coming across the story online. A couple of system administrators are trying to track down an issue in a network, where a particular packet refuses to transmit across the network. The router keeps reporting no traffic, even though they're sending a packet. Eventually, they pull out an ancient router, stick it in the middle and discover that traffic is flowing, but being hidden by an AI that has embedded itself in all the modem devices' firmware.
This sounds like "Coding Machines" by Lawrence Kesteloot.
It starts with a trio of software developers investigating what appears to be a compiler bug. After a lengthy investigation (which is described in considerable detail) they discover that the compiler is "infected" with a mysterious code sequence that modifies programs as they are being compiled, potentially compromising all other programs on the system.
After some discussion, they perform the experiment you describe:
“I don’t know what it’s doing,” said Patrick. “I wonder if it’s sending stuff over the network.”
“We could check that with Wireshark,” said Dave, referring to the program that monitored network activity on a machine.
Patrick guessed what I was looking for and saw the reaction on my face when I had found it.
“I guess we can’t trust it,” he said.
“Trust what?” said Dave, coming back with a soda.
“Wireshark,” said Patrick. “It’s infected.”
“Whoa,” said Patrick. I looked over and the scope was showing a long stretch of activity. I looked back at the switch’s lights, which were dark.
Dave said, “You are not going to tell me that the switch is infected and hiding packets.”
“It is totally hiding packets,” said Patrick, staring at the activity on the scope and putting both hands on his head.
He came back with an old switch, from years earlier, and plugged it in. Its lights flickered in sync with the scope, to packets censored by Wireshark and the modern switch.