How did they find out my iPhone password?

My mum and her fiancé asked to borrow my phone to call someone and when i went to get my phone my mum needed it again and she just typed in my pin code? There's NO way she could've known it before and when i asked her how she knows it, her fiancé said, "you can't hide anything"

how the hell is this possible? I'm so angry right now. I feel like i can have no privacy.

is there away he/she couldve found out?
thanks in advance.

Added (1). like i said, she has not seen me type it before, it does not symbolise anything and there's NO way she could ever know what it is? It just doesn't make sense

Maybe you have used this pin code for previous passwords that your mom might have known about? Do you keep a record somewhere on paper of your passwords? Does your pin code symbolize something? I'm not really sure how she found out… Just try changing your password every few weeks to something different so that your mom can't just unlock your phone.

The only way I can think of, besides her great guessing abilities, is that she has some kind of Watchdog installed on your phone that can watch every move and text you produce.

That or she seen you type it in before.

I like to use stupidly simple pattern passwords on my Android because people would normally think I actually use all the dots to make a password.

Change your password.

Well obviously she knows it so change it. If you have 5S then just use the fingerprint, no way around that.