A few months ago, I installed a power strip that watches the power usage on one outlet and deactivates the other outlets on the strip when that outlet drops below a certain amount of draw. The result is that one monitor's power usage can control the on/off state of all four monitors.
This power strip is pretty handy, since two of the four monitors do not seem to respect the PC's commands to enter power-save mode. Instead, they display a stupid message "No signal" that bounces around the screen until the PC is reconnected. After having looked through the settings for a way to cause the monitor to actually power down and not found anything, this power strip is the only thing that does the trick.
I recently swapped out a flaky main monitor for one that includes some extra features, and in the process, I've made some changes for which monitors are in what positions. The new main monitor has an adjustable stand, which is kind of cool. It also has a built-in USB hub.
After some weeks of not being able to get the main monitor to go into power-save mode (thus triggering the power strip to power down the other monitors), I've finally figured out the problem. The monitor actually does go blank, but the power draw is still high enough to keep the power strip on. The problem is that the USB hub draws too much power, and keeps the strip on.
I was going to use the hub on the monitor to charge travel devices, like my bluetooth headphones, but I plugged in a Sidewinder Strategic Commander that I had sitting in the basement (I'll save the new Minecraft addiction for another post). I suppose that this controller is now drawing enough power to keep the whole strip powered on.
I've moved the controller to a dedicated USB hub (a new one, since my old one also recently burned out -- after writing all of this, I feel like I might have gotten hit by lightning or something), and am waiting to see if that solves the issue completely by itself. I think it will, but if it doesn't, I'll switch which monitor controls the power strip to one that doesn't have a built-in USB hub.
Hopefully if you run into a similar problem, this will give you a hint at what to look.