2000 Pontiac Montana dead battery

We got up this morning and our new van was completely dead. It was a used vehicle when we bought it but we drove it from Kansas to Montana and never had any problem with it. We think my nephew tampered with it and would just like to know if you know what he could've done to cause the car to be completely dead. We checked and there was nothing left on and not only that but the vehicle is equipped to shut anything off after a certain amount of time so that it won't drain the battery. Maybe you can give us an idea of what to look for. It did start with a jump start and we let it run for a little bit but when we went to go to the store it was dead again. We haven't located any cut wires or anything. But you might know something we could look for. We did buy a new battery just on the slight chance that it is the battery. Thanks.