The reason the hiss may stop is that there may be a point when the hardware cuts all power to the audio components when not in use (say a few seconds after using). This could easily also cut power to the thing that is causing the hiss...
Hard to say though, you still could be right about it being software. Almost impossible to know for sure where it's coming from unfortunately
One thing you'll see though is similar problems on all kinds of hardware. I actually have a similar problem with my desktop computer if it has USB devices plugged in near the headphone jack. It just one of those things that can happen with audio hardware sadly
Wish I could be of more help