how would my autofill login/passwords show up on someone else's iPhone?
Hi, The iPhone in question has never been connected to my desktop or iPhone. It hasn't been connected to my Apple ID account. However, we just discovered that in Safari all my saved information is now on this phone... Can someone please help me understand how this happens and what I can do to protect my information more effectively.
That would happen if you sync iCloud Safari data, and/or if you sync with iTunes (either via USB or Wi-Fi). Otherwise, yeah, I don't think there are any other mechanisms for transferring that data along.
"This phone belongs to my child."
Depending on their age and technical ability, I would suspect this is your primary vector for that info getting connected without your knowledge (given everything what you excluded). Especially if they wanted to get something from iTunes, eg, in-app purchases etc.
Would definately not be the first time of seen that happen, I'm afraid.