Had this issue a couple of years back. Some people say that the pin switch under the brake pedal is to blame as you can be on the brakes before the electrical circuit is informed by the pin switch. The switch is easily adjustable.

Get a mate to watch and tell you when the brake lights come on and then wind the pin in or out until touching the brakes brings on the light.

For my car, this wasn't the problem. Mine was caused by the abs sensor wire getting internally fractured when upgrading my rear roll bar. Couldn't really see the damage but internally the wires had been broken by the compression impact.

Subaru wanted to charge some ridiculous amount for the new sensor. Picked one up from the U.S. for about 15% of their charge. About $80 from memory.

A cheap OBD won't see the code but once you put on a Subaru select monitor it will reveal extra codes.

Cheers