Some of the newer stereos have timers that will turn them on and off. Some have weather radios, and if an alert comes in, it will trigger the unit to on. Might have to find the stereo manual and do some reading. If you don't have it, try searching the internet, as most makers put their manuals on line.
And WELCOME to the forums by the way.
2012 Ford F250 ext. Cab 4x4
2002 Jeep Wrangler Sahara
2008 Yamaha V-Star 650 Classic
2008 Work and Play 18LT