Beep noise from front of car or alarm noise

2002 vanden plas. I was eating with a loved one in the back using the trays over looking lake. Great experience. When finished we closed all doors.I pushed the remote to lock doors and it made a beep noise I never heard before and would not lock doors. I then decided to start car to reset whatever it was that caused it. After doing that the remote worked perfectly and locked all the doors. We the walked around lake. Opened doors with remote and drove home. No problem. We do not know why it did that to begin with. Any ideas.