Yes, they can be passed without doing a bona-fide parasite cleanse. In fact, the #1 organ cleanser has turmeric in it, which is highly irritating to them. That's one way to find out if you have them, take lots and lots of turmeric. If things start stirring up a lot - voila!
Sometimes certain kinds of parasites will use gallbladder stones as homes, and sometimes a good colon cleanser will knock the egg sacs loose. It's a good idea to familiarize yourself with all the pictures online here and elsewhere so you know what to look for.