1998 Ford Explorer Alarm

We just bought a used 1998 Explorer. About 5 or 10 mins. after turning the vehicle off and locking up, the alarm goes off. Being unfamiliar with this vehicle, can anyone offer advice on if we need to reset something or ???

We did check the hood button to see that it was in it's correct position thinking that may be the cause, but all is fine there.

Thanks in advance.