If you bring home a new snake, it may not have any visible mites...but within 2-4 weeks you may see them appear because eggs (too small for your eyes to see!) that were laid on your snake hatched and grew up into adult mites.

This is one reason why it is important to quarantine all new snakes away from existing collections. And a reason why many people will pre-treat a new snake's enclosure with PAM even before evidence of mites show up. Kills the eggs you can't see before they ever get a chance to hatch.

EDIT: If you've had your snake for longer than a month, and havent' added any new ones to your collection...it's possible for snake mites to "hitch a ride" home on you if you handled snakes at a show or pet store and then went home and handled your own snakes.