It is under the tank on ALL R3 generations - when an actual alarm is installed it utilizes an extender cable which connects into that main harness connector and the alarm then plugs into the other end of the extender.
As explained in my previous reply, the OP has simply found the end of the extender, not the origin.
It does not really matter if the extender is left in place and the blanking plug connected there under the side cover, where it easier to access, although much cleaner to just get rid of the extender along with the alarm as it is redundant.