So I locked my car the other day, and then I went back into it because I left my iPod. I thought it was unlocked, but it was locked. When I pulled the door handle, an alarm went off. I was unaware that I had an alarm because for the previouse four months it had never happend. I unlocked the door with my keys and it went off. I tried it again, and it din't happen. I was wondering if the 99' 2.5L Avenger came with an alarm.

 

That's your anti-theft alarm also known as keyless entry, lol.
If you like, lock your door and keep an eye on the instrument cluster. When you lock the door with either the key or key fob, you should see a green dummy light that reads...security. If you have that, then you have the factory alarm

 

Also check the manual or glove box for a guide on how to use the alarm. It should be called Auto Mate. If you have one, i would suggest reading up on that. When i lock my car with the remote i make sure all the doors are shut. If you shut the doors first the green dummy light won't come on but the car will still be securely locked. This has usually helped me in preventing the alarm going off. Also if you dont have a manual on the alarm, Count the number of times your lights flash when you lock your car with the button. The number may tell you how high the alarm is set and how sensitive it is. To disarm the alarm to just a regular locking mode, hold down on your unluck button for 3-5 seconds or so. This should help. Setting the alarm is done by pressing the lock button and then pressing it simotaneously afterwards. However many times you press the button within 10 seconds will set how how your sensitivity is. I hope this helps and that i didnt waste my time

Good Luck