NFL Network correspondent Jane Slater shared the story of how she caught her ex-boyfriend cheating while using their shared Fitbit app, says USAToday.

Slater says that they used the Fitbit to track each other’s activity levels and motivate one another. This tracking backfired when the boyfriend’s heart rate suddenly started to spike at 4 a.m.

To make matters worse, while Slater’s boyfriend was out having an early morning workout session, she was home getting balloons prepped for his planned birthday celebration the next day.

One Twitter follower chimed in and admitted that she also caught her ex-husband cheating with the help of a training device.

Professor Alex Koch of Lenoir-Rhyne University tells Business Insider that while a 4 a.m. heart rate spike may be suspicious it isn’t always proof of cheating. Innocent activities like having a nightmare or sleepwalking can increase the heart rate on a tracking device.