ML1500 Power Light

Last week I went to check a PLC that had a fault light on. When I arrived there was a fault light and a power light on. I tried to connect to it with no luck. I told the customer he had a bad PLC, so he ordered another one already programmed from the machine manufacturer. This afternoon I went to change it out for him. When I was done, the new one acted the same as the old one. I started checking and found that I had no 120v power to it. That was what puzzled me. Why did I have a power light with no 120 power? The only thing I can figure is that it was getting it through the analog from the VFD.
Anybody seen this before? It really threw me off seeing a power light when I had no power