Braking Fault alarm over 60mph

[Brake Pad]

I’ve got a Renault Clio that is about 10 years old. I got it service about two weeks ago then I made a four hour round trip when an alarm went off and the dash board lit up saying “Braking Fault”. I completed the trip without any problems and took my car back to the garage. They said they could find nothing wrong apart from the brake fluid being ever so slightly down so this was topped up.

Afterword’s this happened again however if I’m driving at 40 everything is fine the alarm only seems to come on once I go over 60mph. Could this be a sensor fault or something else. I’m a bit concerned because I’m making a five hour journey next week. Should it be okay?