Sorry JohnTr but I don't have a Facebook.
Yes, Fat Homer, I remember that. I had been keeping an eye on them multiple times a day since I brought them home. They had never been up in the tubes and always hung out among the plants and under rocks. They even hung out inside the head of the internal filter, through the cord hole (I used a piece of sponge to block this off though). Anyway, I really had no reason to think they were in the old filter tubes. I feel extremely lucky to have caught them and got them back into the tank safely.
Also, ever since I got them I have kept packing tape over all of the opening on the back of the tank. My new filter uses a spray bar so they can't get into that one while it's off.