I drive my girlfriend's 2010 Prius a lot and know how it works. I usually get about 50 MPG in it. Two days ago on a 100 mile freeway trip I noticed the Eco light never came on, even though I was in ECO mode and was most often driving outside the power zone. Simultaneously the mileage began to drop from around 50 into the low 40s. Something is clearly changed and wrong, so I told my girlfriend to take the car to the local dealer for service. They ran a diagnostic which showed all normal and told her she couldn't believe what she read in the owners manual about when that light should be one. I don't buy it. Can anyone shed any light on this issue. Thanks.