My 2013 Mazda3 had it and the CX-5 i've tested weeks ago also have it.
With "Eco Mode", (wich is auto-triggered and you haven't control on it), the AC system works at low load because the outside temp is nearly the t on the cabin.
For example, here in Santiago, spring or autumn temperature is aprox. 20-23 C outside. So, when you put the climate control in Auto at a similar t, the 'Eco' light appears.
But, when you're on full summertime, 32-34 C or even more and AC needs to operate at high loads, 'Eco' doesn't appear.
It reflects on MPG very high. In fact, the car does 1 or 2 MPG more on all seasons but summer, when the 'Eco mode' triggers.
Cheers.