May 7, 2020
Thank you for posting and welcome to the Bose Community!
This happens due to the Bluetooth Low Energy feature of the SoundLink Color II, the speaker will power on whenever it detects a recently paired connection. It does this to create an easier pairing experience. This isn't a feature that can be disabled on the speaker.
If you have any further questions, please do let us know.
Jun 15, 2018
@Hector_BIs there a way to totally power off the Color II (I don't have one to check) so that BLE is not on, sort of like you can do on the Revolve by powering off the unit by holding down the function key until all the lights blink.
BTW - you failed to let the use know that that a way to stop it from happening is to unpair the device (or reinitialize the unit) before you turn it off. It's a drag but it's a solution.