Flying to Kigali in Rwanda last December meant an overnight flight from Dublin to Addis Ababa to catch a connecting flight on to the Rwandan capital. Airport terminals are never exciting, shops filled with overpriced goods or cafes and bars charging exorbitant prices. Finally, the resistance crumbled and I paid US$5 for a cup of tea. It came with a bonus though, the password of the cafe Wifi. An email was sent home and Facebook was updated with a photograph of two airliners sitting on the tarmac in the warmth of the African morning.

On the return journey a week later, it was dark by the time we reached Addis to connect with the flight that would carry us back to the Irish winter. Taking the phone from my pocket, I was surprised to see it had connected with a Wifi network – the cafe that had sold me the five dollar cup of tea seemed not to have changed their password. Clicking on the WhatsApp  icon, I pressed my wife’s telephone number, it did not answer. Opening the “Find my iPhone” app, I clicked on the link for her phone, it told me she was in the Dublin restaurant Fallon and Byrne. Deciding to wait half an hour, I went to sit with my companions.

“Did you manage to get through to Katharine?”

“No,” I said she is having a meal in Fallon and Byrne.”

“How do you know?”

“The app on my phone told me.”

“What? That’s scary.”

It wasn’t so scary, Katharine and I can only track each other’s phones because we allow each other to do so. It would be a simple matter to turn off the app.

Much more scary is a piece of technology highlighted in last Saturday’s Financial Times. Google now offers a “reverse image search” facility, an extraordinary development that allows one to upload photographs and search for them on the Internet.

Testing it by uploading the original versions of photographs posted on this site, it was astonishing to see them located in less than a second on the pages to which they had been posted. Doing as Jonathan Margolis, the FT journalist, did, I then tested it with a mug shot of myself – it did not fare so well, finding only a number of photographs that were not me.

The technology will improve, though, and it will only be a matter of time before it starts locating photographs of people with pinpoint accuracy. Margolis warns of the prospect of being a woman on a  train who is photographed without being aware that it has happened and who is then approached by someone who already knows her name and everything about her through matches found on the Internet.

The technology will be a boon to stalkers, and to anyone seeking material that might cause embarrassment, and, like all technology, once known it cannot be unknown.

Knowing someone was having a meal in a Dublin restaurant seems like standing at a respectful distance when compared with knowing all the facts about someone’s youthful indiscretions.