Joe-FastFeet There are three key parts to taking a picture with a modern smartphone camera:
- A physical mechanism that adjusts the lens (for optical zoom),
- A CCD sensor array, for capturing light to form an image,
- An app for orchestrating #1 and #2 and storing a picture.
#1 and #3 can make sounds. In the case of #3, most apps have a fake shutter sound as an option, though in Japan in theory disabling the sound is illegal.
Other posters are pointing out that the key step for actually capturing an image, #2, makes no sound. Thus any time a sound is heard it is from something else.
Meanwhile, it is not surprising if resetting the camera device (which happens at boot) causes the lens hardware to move. That does not mean that an image is captured, because adjusting the lens does not capture an image.
Also note that recording video, which captures many pictures per second, does not make many "camera firing" sounds per second. Again, that is because the image-capturing part does not make a sound.
Overall: rebooting the device causes the camera to be reset; resetting the camera plausibly makes a sound; the sound is not evidence that an image is captured; capturing an image does not make a sound.