You say nothing is running, I have my doubts about that. Maybe nothing is open, but if nothing is running at all and it wasn't on, (screen on) when you took it out of the bag my question is, what happens if it's not in the bag?
I used to have this happen sometimes, but very rarely now if at all (I assume updates helped fix it). I use Start Menu/Power/Shut Down in the morning before I take it to work. I put it in the bag power button up, bag kept unzipped so no pressure could be on button. When I get to work, it used to be hot sometimes. Now it doesn't get hot, but sometimes it's on although the screen is off. I open the type cover and the screen turns on and it's at the login screen.
I have it set to come out of standby without needing to logon, and sometimes it comes out of standby (connected standby, I presume) but I've never figured out how it can boot itself (and I'm sure the power button's not being inadvertently pressed).