I usually don’t unplug my speakers (active studio monitors Adam A5X) from the power outlet, but when I do, there is temporarily - usually for a few minutes - a very high pitched whining, almost signal like, noise coming from the tweeters when I reconnect them to the power outlet and turn them on. I don’t know much about electricity, but if I may make a wild guess: could it be the noise/subharmonics of some capacitors being “refilled” after being completely discharged upon being disconnected from the power outlet? But then again, wouldn’t that happen in a few seconds?
I am very eager to learn!
Maybe high frequency pickup on the ground/input causing the internal amplifiers to oscillate. It be EMI from its own internal power supplies. If you have something like LED or fluorescent lighting it might pick up high frequency noise from that.
Could also be the ripple on the power supply increasing as the voltage drops from having no input.
Perhaps try using an app like spectroid to try and figure out what frequency the noise is