What is the most visible low power indicator?
High-brightness LEDs (those with a zillion candela) are noticeable with a tiny current, a fraction of a milliamp... So that could be a good choice. This is thanks to the optics concentrating the light in a tiny cone angle though, which means they lose visibility when you look at them from the side.
Make sure they sit against a dark background.
If they blink for important stuff, okay. If they blink all the time, someone will get fed up of it and stick black tape over it.