Following a previous incident I regular check dds phone. I don't want to go into massive of details but let's just say I have really good cause to do so. She is a young teen with a long journey so needs a phone but was putting herself in a position where she was lying about where she was and who she was with and hanging out with the really wrong crowd.
She is not aware that I check. She was a smart ass when she did know and locked everything down so I could get on nothing. I managed to find out her password.
My issue is she makes up stupid lies to her friends about me.
Some are really daft.
I. E - She made cakes and I wouldn't let her have any or she made cakes but I've ate all but two - I made scones not her and there were loads left 
My Mum insists I am at school for 7.10am and will ring me to make sure.
My Mum is doing x just so she can piss me off. (I'm not doing a course)
My Mum won't let me have people over.
My Mum won't let me go to X toy shop (where messaging friend goes) cause she says I'm too old. My Mum calls me a baby if I do.
But some of it is more serious than that and she caused upset last year with some really quite awful lies but it's all bullshit.
I've challenged her before on this and she says her friends hate their Mum's and she wants to fit in but some of the lies she makes up could get me in trouble!
She's also lying to my face about hanging around with certain people when it's clear she is.
So would you keep quiet and keep checking, stop checking or challenge on what you have found and risk her hiding stuff more.