I was horrified on two counts; firstly that she'd snogged a boy without telling me and secondly because I couldn't understand how her mum could have invaded her privacy in that way.
When I was a teenager it wasn't easy to hide stuff from parents. Mobile phones didn't exist, there weren't even cordless landlines so if a boy rang up, the whole household knew about it!
The development in communications over the last 20 years is awesome. We can email pictures, form relationships with faceless people across the world, meet 'friends ' in chat rooms and generally expose ourselves to millions of people at the touch of a button - often without considering the implications. We hear about grooming, cyber bullying and even naked pictures of teenagers going viral.
What parent is not going to be anxious and more than a little eager to find out what their child is up to - not only when they're out but also in the privacy of their bedroom. Problem is of course at a time when we most need to know the details of their life they become less communicative.
Simple questions like where are you going and with who can be met with incredulous looks of disdain - and if they do suspect you are checking up on them they may stop communicating altogether.
So what are your options? Just about nil unless you feel it is acceptable to invade their privacy by reading their texts, checking their computers or hiring a private detective.
But ask yourself, would you feel it's acceptable for your teen to read your private messages – or have you followed round the supermarket.
I suppose the pro-lobby would argue they are trying to protect their children rather than simply being nosey. And that, according to psychologist Dr Rachel Andrews, is the relevant part. "If parents have a real and genuine worry that their teen could be getting into a dangerous or compromising situation then I believe it is appropriate to find out what's going on. If that involves doing a bit of detective work then so be it.
"On the other hand if you just want to find out if your daughter has a boyfriend then I would consider that an unacceptable invasion of privacy."
I insist my teens are my friends on Facebook – one handy way of finding out if they're 'in a relationship' - a part-solution I'm hoping will last long enough for them to grow up and be savvy enough to be safe.
I realise I don't get to see everything but at least I get to monitor what they are saying in 'public' and that can give you at least some clues as to what is going on in their lives - and if on occasion their Facebook page is left on by mistake well hands up any parent who wouldn't be tempted to sneak a peek.
"I almost can't believe I did it but I opened a Twitter account in a false name in order to find out what my son was up to," admits Maya, mother to George, 17. "I followed his friends first so that it didn't look suspicious. I did get to learn some things but to be honest most things I would rather not have known."
Many would suggest that was going a little far but sometimes it can be difficult to act rationally as a worried parent. "You need to be able to justify your actions," says Emma, mother to Laura, 17.
"A couple of years ago I discovered Laura had self harmed. I knew something was very wrong but she wouldn't open up to me and refused to accept she had a problem. Out of desperation I logged on to her computer when she was at school and found her Tumblr page.
"It was so dark and depressing I realised she was clinically depressed. I made a doctor's appointment for her pretending it was to do with a holiday jab. It took a while for Laura to accept that my 'snooping' had been necessary but I would advise any parent to do the same."
Aside from the safety issues Dr Andrews believes that it's important young people learn for themselves how to manage their life. If they don't get the opportunity to learn from mistakes then they will never will. So although our duty is to protect our children we also have a duty to allow them to grow up and make their own mistakes.
How weird is it though that our teenagers tell us so little, yet the world so much via their Twitter or blog. And of course unlike a diary which can be destroyed, their electronic footprint is there forever.
More on Parentdish: The dangers of teenage sexting