Hiding your phone is certainly suspicious, and there is obviously something being hidden. But in and of itself, that does not necessarily mean infidelity is occurring. They could be planning a special getaway, buying a present, or may have a medical issue they are embarrassed about. So the answer is that it can be.
When you call a dead phone does it ring?
Answer: A: With a dead battery it should not ring but it should go to voicemail directly.