Hi, if Jonnoj's advice doesn't help try to see the wires going through the door to inside the car. Alarm beeping when armed it means a door/s are open, of you have a wire or a set of wires that became faulty and they touch intermittently you will get those errors and the battery will drain. Plus the previous owner knows more than he tells, new alternator, new battery and knowing about the fuse... to me indicates the fault was inspected before, when you went see it, did you find it with the engine running? Hope it helps.