Or even better: find an old ethernet switch, gut it (but keep the connectors) and put Raspberry PI inside. You will need to solder 6 wires for ethernet and power, but the pins are fairly large so this should be easy.
Even if discovered, most people would not bother taking it apart --- they'll just assume it is broken and throw it away.
This is exactly what I was thinking. Even the network admin would probably be like, "well, I don't think so but I'd better not mess with it, just in case it's how the CEO is getting internet". Unless of course they engineered the network originally.
I have a 4 outlet "surge protection" power board with a Pi Zero W, and USB power supply, and 4 240V mains relays and drivers all neatly tucked/hidden inside... I use it as Wi-Fi controllable power points, not for pen testing, but at this stage that's just a software update...
Check out the image in the article.
They attached keyloggers and sent the strokes to the box. Saving them and once in a week dump them over to a car in the parking lot.
The original article is great, but the guy was really not putting any effort into it.
A high voltage warning sticker is likely to gather a lot of attention, especially inside a network closet.
There are many rules related to where high voltage stuff should be, how it should be installed and who can access it. And unless you do it by the rules (unlikely), it will get caught up during a safety inspection.
Even if discovered, most people would not bother taking it apart --- they'll just assume it is broken and throw it away.