It's the alarm for sure. It's the same sound when you shut the trunk and the shock warning-3-5 quick horn beeps-happens.Are you sure its the alarm and not the front passenger side airbag seat sensor going off if you have something sitting in/on the front passenger seat?
I have an 05 LE. Don't recall them coming with a factory alarm however, just the engine immobilizer and the blinking "security" LED in the cluster left of the steering wheel. When ever I put anything that is over 7 or 8 pounds onto the passenger seat the seat belt chime goes off briefly and the air bag light for the passenger goes on. If the weight shifts enough to trip the sensor off then next time the sensor trips on while driving I get the brief dinging sound again. Learned very quickly not to put my laptop bag in the front passenger seat. Drove me nuts on long drives.