The thing that gave him away was a notification the size of a postage stamp.

It was a Tuesday, the kind that has no edges to it. Daniel had gone up to bed early with one of his headaches, and I was on the sofa with his old laptop because mine had died and I just wanted to check the bank balance before the mortgage went out. I wasn't snooping. I want that on the record, even though there's no one left to say it to. I was paying a bill. That is the most ordinary sentence in the world, and it is where my marriage ended.

A little grey box slid up from the corner of the screen. He was still signed into his gaming account on that machine, the way he was signed into everything, because Daniel trusted our home the way you trust your own hands. The box said he had just started playing something. A new title. I recognized the name because he'd mentioned wanting it for months, and I'd mentally filed it under "too expensive, maybe his birthday."

But his birthday was in March. This was October.

I should have closed the laptop. Instead I sat very still, the way you do when you hear a noise in the house at night and you're hoping it's nothing. Then I clicked through to his profile, because some part of me had already started moving before the rest of me agreed to it.

The game was there in his library, shiny and new. And next to it, in the small print that nobody is ever meant to read, was a line saying it had been a gift. Received eleven days earlier. There was a little message attached, the kind you can leave when you buy something for someone, a digital ribbon on a digital box.

It said: *For my big strong man. Enjoy, baby. xx*

I read it maybe nine times. I kept thinking the words would rearrange themselves into something I could survive. My big strong man. Daniel is five foot eight and apologizes to furniture when he bumps into it. Nobody who shares a bathroom with a person calls them that. That is a phrase from the beginning of something, from the giddy part, the part we'd had once and then traded for a calendar on the fridge and a rota for the bins.

The sender's name was a handle I didn't know. Letters and a number. I clicked it. A cartoon avatar, a profile locked down to almost nothing, but not quite nothing, because people are careless when they think they're invisible. There were public comments between the two of them going back months. Inside jokes I had no key to. A little heart he'd left under something she said. The casual, fluent intimacy of two people who talk every single day.

I sat there until the laptop screen dimmed itself and I had to touch the pad to wake it, like I was keeping vigil over a body.

The strangest part is what I felt, which wasn't rage, not at first. It was a kind of awful tenderness, almost grief, watching my husband be charming and warm and attentive to someone, and recognizing every move of it, because he used to be that way with me. He hadn't lost the ability. He'd just relocated it. He still knew how to make a person feel like the centre of the room. He'd simply decided I wasn't the room anymore.

Upstairs the bed creaked. He was awake. Probably texting her in the dark while I sat in the blue glow of the proof, holding it in both hands.

I didn't go up. I made tea I didn't drink and watched the steam come off it and thought about how the platform had done this so innocently, so cheerfully, a feature designed to let friends share their fun, broadcasting his secret across the house like an announcement at a station. He'd hidden the messages, the second phone, the late nights "at the office." He'd thought of everything a careful man thinks of. And then a piece of software, trying to be friendly, simply told on him, the way a child does, without malice, without knowing.

When he came down for water at one in the morning he found me at the kitchen table with the laptop open and turned toward him, the message glowing on the screen.

He stopped in the doorway. I watched the strong man go very pale.

"Enjoy your game," I said. My voice was perfectly steady. It frightened both of us.

I moved out within the month. People keep asking if I wish I'd never seen it, and I don't. I'd rather know who my husband became than keep loving the man I'd invented out of habit. The pop-up didn't ruin anything. It just delivered the truth that was already sitting in my house, addressed to me, postage paid.