This isn't all that abstract or theoretical of a problem. There's lots of devices (printers without lcd screens, routers, for example) which use one or two lights, possibly with different color options, to display a bunch of different possible errors. Often this simply means creating a "key" for the interface (i.e. check the manual to look up what two blinking red leds means or what one steady green and one blinking red means) which is admittedly not fun for the user, but it's also damn hard to make things intuitive with so little resolution (literally!) to convey your info in.
My favorite example of this is how the linux kernel will actually give you morse code through the keyboard lights (num lock, scroll lock, caps lock) during a kernel panic [0].
Cool, there is a scene in Neal Stephenson's Cryptonomicon where the protagonist programs his keyboard lights to play back messages to him in Morse Code. I had no idea it had been implemented IRL.
Back in the early days of CHDK (custom firmware for Canon cameras) they would dump the original firmware code of new cameras by writing a tiny loader (that looked like a firmware update program to the camera), put that loader on a memory card in the camera, run the loader on the camera via the firmware upgrade menu option and that loader would just read the original firmware out of memory and blink an LED on the camera over time to send out all the bytes of the original firmware. A computer would then capture that signal with a photodiode connected via serial port or sound/mic input resulting in a binary dump of the camera's original firmware on the remote computer where it could be disassembled and reverse engineered and (eventually) extended to have all sorts of functions the camera didn't ship with.
If your computer starts acting visibly funny (as apposed to funny in the sense of weird processes or network activity), you can almost entirely rule out a virus, those like to stay hidden.
That's an interesting observation actually, because it didn't use to be the case. It used to be the case that most viruses would intentionally make a nuisance of themselves or display messages every now and again (e.g. you might get a message on boot every 10 times or so - too often and they would get eradicated before they spread).
Viruses that are intended to stay hidden and undiscovered is a new thing, relatively speaking.
