I'm guessing the intention here is that they're fake, marker drawn ones. Still not great, but I'd think the artist who made this would at least understand that much if they understood rules like no phone, no pics, no talking to pigs.

The commonality of symbol itself isn’t necessarily the point. If you have one that is easily identifiable (such as the anarchy sybmol) and the location is readily apparent, say on your left arm on your elbow, then that’s enough for them to go with to help with ID the person. While there may be plenty of the symbol around, the odds of a person of a certain age/sex/color having that same symbol in the same spot is pretty close to zero.

Late edit: if you want to fool visual identification via tattoos, get fake tattoos that are visible and then remove them.

Cover them, and don't get arrested, because if you get arrested, they will strip you naked in order to force you to shower and make note of any tattoos you didn't divulge in booking.

I don't remember where I read it, but apparently the rock in shoe thing doesn't work all that well.

Basically, yes. Plus, it may be unconstitutionally searched if you are detained.

Yes. Even when your phone is powered down, some models still ping cell towers. If it pings one, they know your distance to the tower. If it pings two, both towers know your distance, and the overlapping circles would reveal two positions coordinates, one of which you were at. With some contextual information, it's easy to know/prove which one you were at.

If it pings 3 towers, your exact location, and unique identifying information sucha as your phone's IMEI is revealed. So don't bring a cell.

The mass of people chanting and holding signs is usually enough to draw the required attention. That's kinda the point.