Wireless connections can be the culprit
If you have peripherals or other devices connected to your Mac Pro via Bluetooth or WiFi, and those devices are on at times when the computer is not, there is the possibility that these devices are waking your Mac. I had this issue once with my Mac, and discovered that one of my printers was the culprit. I wasn't thinking of turning off the printer because we share it in the house, and just because I wasn't using it didn't mean that someone else didn't need it. I discovered that, when I turned that printer off, my Mac stopped waking unexpectedly.
Check around. You may find some BT or WiFi connection is nudging your Mac Pro into wakefulness.