98 DeVille, door key, locks, and alarm

Not a lot of info yet but trying to help a neighbor with a 98 DeVille.

With the key in the door lock -
1) Sometimes the door key will lock the doors, sometimes not.
2) Sometimes the alarm goes off, sometimes not.

They have something to do with each other but not sure what yet.
Any hints on narrowing down the problem?
The key? The door lock?
Will see if he has any dash board messages.
Can the alarm horn just be disconnected?

Not much to go on but thanks.
Hope to have a bit more info soon.

Just an update - hope it helps a bit.

No remote entry - just keys.
Once in a while the door key will unlock just the driver's door instead of all four doors.
If the driver's door is the only one un-locked then he gets the alarm when he opens it and then needs to lock and un-lock the door to silence the alarm.

Aha !
A quick turn of the key in the door lock just un-locks that door.
Keep the key turned in the door lock for a few seconds till all four doors un-lock.
Need all four doors un-locked or the alarm will sound.

I traded in my 98 DeVille for a 08 Lincoln Town Car (big mistake) but after putting 124,000 miles on the Deville I know how the locks work. Assuming the doors are locked: operating the key once opens the door with that lock, operating the fob once opens the driver's door, and operating the key or the fob twice opens all of the doors. The alarm will never operate when door(s) are unlocked with the key or a fob and then opened. This seems to be standarized because the Town Car operates the same.

Starting the car or locking and unlocking the locks with the fob will silence the alarm. The Town Car is different because the alarm can only be silenced by starting the car, which is one of the dumbest ideas ever. Even dumber, the alarm goes off in the Town Car when a door is manually unlocked from the interior by pulling up the manual lock.

If it is not working that way, take the car to a dealer for repair.