It might have increased its transmit power since it couldn’t receive any tower signals, and so it killed its battery much quicker than normal.
Incidentally you can test the effectiveness of a Faraday bag by putting your phone into it and then calling it from another phone. If your phone rings then it’s still in touch with the local cell tower, which means the bag isn’t completely blocking signals.
The battery was approx 50% charged when I removed it.