Change in sleep behaviour on MacBook Pro and ignoring settings on Yosemite

I can't tell why you Mac wakes up, but here is how you could find out:

Type pmset -b -g assertions in your terminal. That will list the drivers that have requested power setting overrides when running on battery.

You can also inspect the power management log, using pmset -g log.

For example, on my 10.10.5 MacBook Pro 2015, I found out that leaving my Sandisk Ultra fit USB3 memory stick plugged in makes my Mac wake up every 3 hours. Here's why:

$ pmset -b -g assertions
2015-10-19 18:38:32 +0200 
Assertion status system-wide:
   BackgroundTask                 0
   ApplePushServiceTask           0
   UserIsActive                   0
   PreventUserIdleDisplaySleep    0
   PreventSystemSleep             0
   ExternalMedia                  1
   PreventUserIdleSystemSleep     0
   NetworkClientActive            0
Listed by owning process:
   pid 51(powerd): [0x000000040008012e] 03:37:52 ExternalMedia named: "com.apple.powermanagement.externalmediamounted" 

ExternalMedia is set to 1 by process powerd.

Unfortunately I haven't found out yet how to prevent this. I looked for a "com.apple.powermanagement.externalmediamounted" plist file but could not find any... So I don't know if these driver overrides can be changed or disabled. :( Anyway, I expect it is driver dependent, so changing the behavior should also be driver dependent...

This detailed web page seems to indicate that, for external media altering sleep behavior, the only option is to unmount or eject the external media. Based on the driver the external media uses, eject is sometime required while for other unmount is enought.

My Macbook Pro 13-inch (2012) also started to show weird signs. When I close the lid and open it after 5 minutes, it wakes up normally. But if I put it to sleep for an hour and wake it, my DVD drive would make a noise like on startup boot. I do not see the overexposed screen with a loading bar like after hibernation. So there is definitely some new kind of power sleep that seem to shut off power to internal devices. I have "Put hard disk to sleep" option unchecked too. And as you mentioned the Power Nap is not supported on this model. Does seem to be a new Yosemite setting.

EDIT - Although, I must admit I noticed that the battery now will last longer once my Mac is asleep for a while. After a few days in a sleep mode, I used to lose some substantial amount of power. Now it is just a few %. So I guess whatever they did, does help to preserve energy.