There exist special UPSes that you can attach to a power line of a server, unplug and unmount it, and keep the server powered all the time, without the server being able to detect that.
If you are really serious about security, you need to have a watchdog on network link downtimes because, no matter what, you can't disrupt a fiber optic network link without the server noticing.
I'm having trouble remembering what they're called, but I do remember reading about these. I saw a demonstration where they slightly pulled at the desktop power plug in order to hook a thin device around the plug connectors to provide power, and then they could transport the desktop around.
Actually not because a server with multiple PSUs definitely knows when one loses power. It’s made to raise alerts about a power rail failing. So this makes it harder because you need to keep power on both to not raise any alarm
If you are really serious about security, you need to have a watchdog on network link downtimes because, no matter what, you can't disrupt a fiber optic network link without the server noticing.