As it's something that is not used on your car, as your original box didn't have it, I would suggest it is a additional speed sensor, from a later car, most likely the road speed sensor the box/ecu uses as opposed to your speedo, reading sensor, which may not even be electronic anyway, doe sit have a speedo cable ?
What ever the case it doesn't alter the leak, simply take off the sensor and re seal it, that's all that's needed.
tabetha