If my speakers are in sleep or idle mode, why can I still hear a faint hum or hiss if I put my ear very close to the speaker?

Even though the final amp stage is shut down during sleep, the pre-amp stage is still powered up so a slight hiss is not uncommon. If you have an analog audio cable plugged in to the speakers but not into anything else, it’s also possible this could be picking up some noise as well.