“How’s Nick?” “I don’t know, I haven’t seen him for three days.”

I’d asked my friend’s partner about their teenage son, who was staying with his girlfriend. “At least I think he is,” he said.

One of the many challenges faced by parents of teenagers is keeping tabs on them.

“Where are you going?” “Who with?” “What will you be doing?” When will you be back?” It’s like an interrogation scene from Prime Suspect – and the answers are usually the same as you’d get from seasoned criminals: “no comment,” “no comment,” “no comment.”

Of course, for parents, one of the advantages of having teenagers is having more independence yourself. They come and go more or less as they please, get buses and trains and generally go their own way, leaving you to go yours.

It is liberating after years of ferrying them around and hovering in the background.

But teen freedom has a downside – because you still need to know the where, who, what and when details.

When they’re going round in a group, it isn’t easy to keep track of who is at who’s house, how long they’ll be there and – if it’s not on a bus route – how they’re getting home.

“Be back by 9pm,” I’ll tell my 15-year-old, and as soon as the clock moves beyond that I start to fret. Ten minutes later I’ll ring her, knowing it to be a waste of time.

You’d think with 21st-century technology, that keeping in touch with your children would be relatively easy. But no, it is too much to ask of a teen that they A: Turn on their mobile before leaving the house, or B: Answer it when it rings.

“There was too much noise, I didn’t hear it,” “It was at the bottom of my bag,” – are among the excuses for failing to pick up.

Worse, are the occasions when I call my eldest daughter and hear the phone ringing in her bedroom. “I forgot to take it with me,” she will say.

It’s hard – I don’t want to come across as over-controlling, but at the same time I want to know they’re safe.

I’m dreading the time when boys come into the equation, turning up at the house at all hours in cars with giant spoilers and exhausts the width of dustbin lids. Then, there will be only one course of action.

I’ll do it in the dead of night when they’re asleep. They’re bound to notice next morning when they struggle to get their tights on, but electronic tags will set my mind at rest.