It's aftermarket.

 

Yeah its definately a proximity sensor for an after market alarm. Thats the most common place for one in an S2000.

 

It's a clear plastic box with the only identifiers:

FCC ID: EZSDEI508
508D

I locked the car with the Honda key fob, left the window open, waved my arms into the car, reached in and unlocked the car from the inside (while being outside) and no alarm was triggered. Is there a separate "arm/disarm" remote that I am missing? Is there some way to find out how to get a replacement alarm control if necessary?

Edit [new info]: I found a generic alarm (proximity, etc.) with the 508D designation, but in the "for sale" picture listing, it only shows the box (looks just like mine) and some wires - nothing for arm and disarm (no apparent key fob).

Does anyone have one of these? If so, how is it armed and disarmed? It doesn't seem to be connected to the remote entry system which works fine - unless the sensitivity control is way off the charts toward insensitive.

 

From the fcc id it is a part of DEI/Viper alarm, a Field Disturbance Sensor

https://fjallfoss.fcc.gov/eas/GetEas731Repo...%27EZSDEI508%27

EDIT: Check in the driver foo well above the fusebox - that's where installers usually "hide" the main brains.
Honda wraps OEM harnesses with duct tape - if you see bunch of regular electrical tape - you're on the right track for finding an aftermarket box.

If this is your sensor:
http://www.amazon.com/Dual-Zone-Field-Dist...I/dp/B0009ILFU2 ?

You should be looking for a box that looks something like this:
http://www.amazon.com/Viper-3002/dp/B000NP...4351210&sr=1-22

 

Yep, that looks like it. Many thanks - I'll go looking for the rest of the system. Unfortunately, we didn't get the arm/disarm fob with the car. We didn't even get the factory fob - had to buy one and get it programmed by the dealer.

I may need to see whether I can just buy a fob/key and get it programmed for the existing alarm.

 

It will not work with the factory alarm. If you go to a DEI brand Dealer/Installer, they should be able to locate and identify the alarm "brain". They will be able to order new key fobs for it and program them to it.

 

The only "brain" module I've found so far is the Omron G8D-452H-B. I assume that this is the factory keyless entry module. Am I correct in assuming that this is not an alarm module?

Keyless entry seems to work fine but no alarm features work - either opening a door after it is electronically locked or reaching through the window and handling the proximity sensor.

So I guess that I haven't found the alarm "brain" yet.

 

You know where the proximity sensor is. You can just follow its colorful 4-conductor wiring back to the aftermarket alarm.