Well since the ABS is a magnet and pickup system, and you had the brakes recently done when it first popped up; I would check all the pickup on the wheels and see if there's any residual metal hanging onto the pickups. This would cause signal interference and throw a code.

The code 18 is for "accumulator gas leakage" Which if I remember correctly this refers to a gas cylinder mounted onto the ABS block, you can't miss it. It has 2 o-rings on it that can fail.

An older model, but same idea.