What could have caused car battery to drain? I think service department not being truthful.?

Maybe the place you bought it from wasn't truthful when they said it was new. Anyways, something was definately using power, or your battery wouldn't be dead. The radio doesn't work with the car off unless the key is in, and the power is switched on. The headlights tail lights, and dome light all work without the key in, amongst some other features your vehicle may or may not have. If a light is burnt out, then it won't draw any power. You 100% sure the light wasn't on? Was your door locked? Maybe someone pranked you and opened your door for a few hours during the night. A bad connection can make power drain fast, but not without the power being used.

I like these ones.

"I took my car to the dealer. There was nothing wrong when I took it in. Now there is."

That's not true, there WAS something wrong, That's the reason you took it in to begin with. At a guess, the dead radio is still drawing power? Something shorted out inside maybe?

But that's only a guess. If you want a 2nd opinion , take the vehicle to a specialist auto electrician and pay him to check it over. He can confirm if there is a power draw, and if it's the dead radio, that can be isolated (remove the fuse). You will have to pay for his time of course, but at least then you KNOW what the problem really is.

Drive it backwards at light speed for five years and it'll be a brand new car at the end of your journey.

Good Luck!

just a thought do you have a light in the trunk? if so check to see if that light is staying on