All those ghost hunting shows on tv are undeniably, regrettably, and amusingly fake. Overhyped, hammy, and quite often wrong... but the principles behind spirit photography are sound. This is where SM comes in.
One of the first ways in which people notice SM is in pictures. This is due to saccadic masking, the camera picking up what is too fast for the mind to register. Saccadic masking explains the way you are selectively blind for a few seconds every time your eyes move or something else moves too fast for your brain to make heads or tails of it. If not for saccadic masking, the vertigo would probably knock us on our butts every time we focused our eyes on something new.
Cameras, because they are snapping an image of the most infinitesimal slice of time, have a better chance of picking up what the eye and brain misses. My mother has an incredible ability to take pictures of ghosts and even to call them into a picture. If I ever decide to scan any of her pictures onto the computer, you'll see our backyard literally crammed with orbs and mists. I have photo albums full of her pictures. She doesn't see anything when she takes the pictures, but there they are nonetheless.
SM is like a ghost in this respect. Some people say that the reason His movements on video camera are so odd is because He is jumping through dimensions or time traveling. These are both valid theories, but it is also quite possible that He is moving in some way that is not only so fast that it triggers saccadic masking, but that it is sometimes even too fast for the camera.
At first He appears very still, which may be as much to acclimate us to His presence as to camouflage Him from casual observation. The more we see SM, the more our minds begin to adjust to His movements, picking Him out and exempting Him from saccadic masking. The adjustment can hardly be smooth, which may explain some of the physical discomfort (nausea, vomiting) involved. It may even be that once He realizes He's been noticed, He makes an effort to slow down to capture more of our attention.