B5 leaking water

[bigkoll]

hi all, looking for pointers if possible my 20ltr passat is throwing up a warning of
"stop check coolant" but no rise in temp on dial no loss of power.

upon investigation i found water level low and topped it up but it happens now and again, i have now noticed a leak from a pipe directly at the rear of the engine with a sensor attatched and i dont know what this is.
any help appreciated.

thank you in advance.