A pretty failproof test would be to:
- Fully charge battery, and verify it's 100% before proceeding
- Turn your phone completely OFF (Android = long-press on power button, then
- Leave it sit in "OFF" condition for as long as possible - at least
- When you power it back ON, then observe battery charge condition. If
it's more than 1-2% lower than when you turned it OFF, then your phone's OFF
isn't really OFF, as there should be zero power drain in that condition.
For any "hidden background processes" to be running and be able to "listen in" (or perform other unauthorized monitoring), you would still need to keep the cellular radio powered ON to listen for a remote "wakeup" command.
The cellular radio is one of the most power hungry parts of your phone. You can verify this by enabling "Airplane mode", which turns off all "radios" in your phone... cellular, Wifi and Bluetooth. If you do this, and leave your phone sit idle for an extended period (12 hours), it will throttle-down the CPU, and use almost zero power... expect to see maybe 3-5% usage over 12 hours of inactivity.
This is very useful for using the alarms overnight, without running the battery down if you don't have access to a charger.
You might think "airplane mode" would be as "safe" as turning the phone OFF... but it's not... because (as others have mentioned) as long as the CPU is still running, malware can still run and force the "radios" back ON at-will... but if the CPU is not turned ON at all (Power = OFF), nothing can happen.
The concept of a heartbeat "wakeup" mode isn't very realistic either... since if the device is truly OFF, loading all of the OS is still required to be able to run any real process... not much happens without loading the OS... which takes quite a while - with respect to a "heartbeat" type hidden process.
Until phones are designed with a "static-state" (whereby the complete state of RAM and CPU's are instantly resumed upon application of power)... so that the OS never needs to load, you can't really use this type of heartbeat process.
(this type of design is exceedingly expensive... seriously)
25+ yrs of EE design experience speaking here... and in consumer products, it's just not feasible...