syncup tracker, and snycup drive

I bought and installed a syncup drive in my Yukon that I let my daughter use, I made sure to hide the port way up in the dash to make sure it wasn't visible. I installed a syncup tracker in my truck that I let my son use I hid it in the dash completely out of site.  They both found their trackers in my cars. Their phone told them there were unregistered trackers near them. Their phones helped them locate the trackers. My daughter got all the way up in the dash and unplugged it and gave it back to me.  My son showed me the notification he got from his phone. But he can't remember where the tracker is now.  I've search the last known location and tried to make it make noise but the battery is dead. My children do nit share phone account with thuer abuela pays for thuer phone. They did not know that i purchased these trackers or installed them. How did my children devices discover my trackers? How do I make my trackers invisible so they won't find them again?