I've been away for about a week a couple of times and had no sign of problems on returning to my Prius. That said - I have the model without the 'smart' key.

I've heard from other drivers about other problems with the smart key, perhaps it's not so smart after all.

A possible issue is where you leave the key, it seems that the car can detect the key at a much greater range than it operates, and if the key is close but not near enough, eg in a room next to the garage it keeps checking the distance to see if it should activate.