coolant sensor

[turbo boy]

Well after getting my rad re welded and new pipe attatched there is no more leaks but still the coolant light comes on. Where is the coolant sensor? How do i get to it?
 Cheers

[turbo boy]

Underneath the drivers side headlamp there is an over fill water tank with 3 holes. one pipe goes directly to the ast, the other is connected to the long upright coolant tank that sits right next to the headlamp and there is one tiny hole with no pipe coming off it. This is where the water is gushing out when there is enough pressure.
My question is, do i blank it off with a rubber bung?
Or is this where the coolant sensor goes? There is no over leaks just this tiny hole with no pipe coming off it.  
Help me out dudes you no i loves ya;)