Car Alarms: Built in alarm, mitsubishi diamonte, spark 2

Question
my 1999 mitsubishi diamonte has a built in alarm that is activated/deactivated with the door key (my keyless alarm has not been in use for years) but recently when I lock my doors, without warning (a while later) my alarm will sound....there has never been any compromise of the car in any way..it just goes off...what could be causing this to happen?

Answer
Please be more descriptive if possible.

"GOING OFF?"

I need to determine whether it's a factory alarm
issue or, if it's an aftermarket system you are
dealing with.

Ok let's clear up the terminology glitch here.

1. FACTORY/OEM alarm = Part of the original schematics
and plans from the manufacturer.
(horn honk, head lights flash, Starter may crank and
die, no spark)

2. DEALER INSTALLED alarm = Just that. Once the vehicle
is brought to a dealer, the dealer sub contracts the after market
security out.
(horn might honk, siren and parking lights flash, starter kill relya,
no start, no crank, just dead.)

3. AFTER MARKET alarm: Same as #2 but Customer IS AWARE that
the system was put in at the dealership, but may not
be informed of the brand name, or even what it's
total capabilities are.
(siren & parking lights flash, starter kill)

Which one applies to yours?

RM/ALARMTEK
CARALARMMAN.COM