Because it was on while in the bag it was using more power searching to connect to a cell tower.
Absolutely correct.
One has to remember a smart phone is really little more than a small computer with a range of radios and antennae.
If it’s an Android, just power it off, completely. At that point it is an inert brick, safe from all but advanced nation-state very near field communication hacks - like holding it against a task-specific RF exciter.
If you power off your android and place it in a REAL Faraday bag (and there are specs for this) not just an anti-staic bag, then it will NOT discharge the battery other than natural power loss over a LONG time.
If you just put your android into suspend, perhaps by depressing the power button on the side to lock it and black the screen, then one MUST first turn on airplane mode, AND turn off Bluetooth AND NFC (near field communications). Airplane mode on some phones only turns off Wi-Fi and cellular radios. NET: turn on airplane mode, then go into settings and turn off Bluetooth and NFC. Of just power it off - far simpler.
The same applies to an iOS device, but getting it truly powered off requires diligence.
The carriers CANNOT turn on a powered-off smart phone. Period. Neither can a Wi-Fi network admin. There is a capability analogous to “wake on ring” for networked PCs, but to PREVENT this, don’t put your phone into suspend (aka locked) - just turn the d@mn thing off and put it into your TRUE Faraday bag.