The thing nobody tells you about catching a liar is how boring the evidence looks before you understand it.

It was a phone bill. Not a lipstick, not a hotel receipt, not a stranger's earring under the seat of the car. Just a spreadsheet of grey numbers that arrived in my inbox the way it did every month, and that I usually deleted without opening. That month I opened it. I don't even know why. Daniel had been a little too cheerful lately, in that brittle way people get when they're carrying something, and I'd learned to read his moods the way sailors read the sky.

I pulled the detailed version, the one that lists every call, the time, the duration. And there it was. The same number, over and over, marching down the page like a heartbeat. Nine-fourteen in the morning. Ten-forty. Twelve-oh-five. Two-thirty. Four-fifty, just before he drove home to me and kissed the top of my head and asked what was for dinner.

Some of the calls were ninety seconds. Some were forty minutes. They started the day he got to work and stopped the day he left for the weekend. I sat at the kitchen table with the laptop and my tea going cold and I counted. He had called that number more times in a month than he had called his own mother in a year.

I want to tell you I did something clever. I didn't. I have read all the stories about the apps and the trackers and the second secret phone taped under a drawer. I didn't have the stomach for an investigation. I had a number, and I had two thumbs, and at eleven o'clock on a Tuesday morning, while he was at his desk a mile away believing his life was airtight, I simply called it.

It rang twice.

A woman answered. Bright, busy, the voice of someone who answers phones for a living. "Good morning," she said, and named a shop. A little homewares place. Candles and soft throws and the kind of overpriced ceramic mugs you buy when you want to feel like you have your life together.

I knew the shop. Of course I knew the shop. It sat directly across the street from the building where my husband worked. I had walked past it a hundred times. I had bought him a birthday card there once, from a rack by the till, while a woman wrapped it in tissue and told me to have a lovely day.

"Hi," I said, and my voice came out steadier than I had any right to expect. "I'm so sorry, I think my husband has been in touch with someone there. Daniel?"

The pause told me everything before she did. It was the kind of silence that has a person in it, holding their breath.

"Who is this?" she asked, and the brightness was gone.

"This is his wife," I said. "Mireille. Who am I speaking to?"

"Niamh," she said. Just her name, surrendered like something dropping out of a pocket. And then, smaller, "Oh, God."

I didn't shout. I think she expected me to. I asked her how long, and she told me. Four months. I asked her how it started and she told me that too, the words tumbling out of her now, relief and panic braided together, the way confessions go when someone has been waiting a long time to be caught. He'd come in for a candle, a gift for me, as it happened. They'd talked. He came back the next day for nothing at all. After that it was the phone. All day, every day. The shop was quiet in the mornings. He had time. They both had time. They had filled it with each other while I filled my days believing I was married to a faithful man.

What undid me wasn't the affair. It was the geography of it. I kept picturing the two buildings facing each other across a narrow street, and my husband standing at his office window, looking down at her doorway, both of them on the phone, separated by forty feet of pavement and held together by a wire. So close they could have waved. So close that for four months he had been, in every way that counted, having an affair on his lunch break and walking back into his own life by dinner.

I thanked her. I genuinely did. She started to cry and I told her, gently, that she didn't owe me her tears, she owed me nothing, we were both just women a man had lied to, and then I hung up.

I didn't confront him with drama. When he came home I had the phone bill printed and folded on the table, and the name of the shop written across the top in pen, and underneath it one word: Niamh. He saw it from the doorway and stopped dead, still holding his keys.

"You called all day," I said. "I just called once."

He left the marriage in the same way the calls had ended on that bill, abruptly, mid-sentence, the line going dead. I kept the printout for a while in case I ever doubted myself. I never did. Some betrayals you have to dig for. Mine had been sitting in my inbox every month, patient as a clock, waiting for the morning I finally picked up the phone.