On the windows 7 side, check the "power properties" , "change advanced power settings", "Sleep" . "allow wake timers" , "disable".
One more bios item, the "start back up on power loss" (by whatever name they use). That switched on can restart the computer after a power loss. The assumption here is that it lost power , which may look like it came out of standby. power fluxes could possibly trigger a wake via a device but it would be rare.
As paul mentined there are a few things in task scheduler in windows 7 that can awaken the computer to do thier things, but usually would allow it to go back to sleep when done. Also other software that is allowed to be run at startup, that starts looking for updates. There are ways to insure that nothing can ever wake from standby, when it comes to software.
A mouse that moved a pixel, more than one time in my working with computers an optical mouse was just in the right spot on a poor surface for optical movement recognition, and it kept the computer from going into standby. Speaking of mices, cats , dogs , kids, a breeze, or even a heating duct that might trigger a wake event via the mouse or keyboard. Testing that can go as far as removing both temporarily.
In Windows again, the event log would be a most valuable tool. it should show any restarts, any wake events. If you knew what time it occured, you could just look based on time also.
In a command prompt , try this
powercfg /Wake Timers I have no idea what it does, because I have none, there are other fun options for powercfg so be sure to try powercfg /?