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.