Why did the alarm work without sound and vibration?

Question

iOS

iPhone SE, firmware version 10.2.1.
The alarm clock was set for 6:45. I woke up myself at 8:45, while on the phone there was a screen of a working alarm clock (buttons "Later" and "Disconnect"), but there was no vibration or sound. The phone was NOT on silent mode (what is set by the side button), the sound was NOT at a minimum (volume buttons on the side).
What else could be the issue? Despite the fact that it was not possible to reproduce the problem.