It's most likely the O-ring/washer in the sensor went bad. It's a 10cent piece of rubber and that's being generous. Unless you can match the exact O-ring/washer it's $50 for a new sensor. That was probably the cause of your original "leak" unless the tech really found a leak in the radiator.

The sensor just screws into the expansion tank and monitors the level of fluid. Your fluid just leaked through that hole.