The boot print could have been there from before the snake even got there.
They don't always bite if you get near them. Depends on their mood, and how hot / cold they are. One of my friends surveys and they go by rattlers all the time, one time he looked down and the thing was literally about an inch from where he stepped, it didnt bother him a bit but nonetheless he moved on. Most the time they wont bother you unless you bother them, ie kick or step on them or make threatening moves at them. If it's sunning itself and you startle it now, then it could lash out.
still, the best rattler is a dead one imho
aaron