Building a network tap into a CAT5 cable isn’t that hard you just need to essentially modulate the traffic into RF and have another implant near by that can intercept the radio signals.
We live in a day where we have demonstratable side channels attacks against RSA keys by listening to how a laptop squeeks when it’s under load and you think building a chip into a cable is a stretch?
Heck I have one cable like that atm which is a converter from a model M keyboard to a USB you can’t see the converter IC it’s built into the RJ45 connector the keyboard originally used without any additional bulk.
I’ve seen a demonstration of similar taps on VGA cables that transmit the entire image to a remove reciecer which is often implanted near by in a power socket, light fixture or anything else where you have a reliable power supply and enough wires to hide and even transmit a signal out side over the power lines.
All of these taps rely on external transceivers that will either record the traffic for later extraction or exfiltrate it through other means.
Also while PoE has more power it’s also more sensitive to voltage drop over the line which means the tap it self will be detected while normal Ethernet works from -+0.5v to -+2.0v without any issue the voltage range is to allow for voltage drop over longer cables so any drop would be ignored by both end points as they’ll just assume the cable is a few meters longer.