A Line Mechanic was informed the MD-82 aircraft he cleaned and removed a 'Blue' streak from the forward Lavatory exterior Service Panel returned later with a new leak at the same location. Lack of any required documentation in their Logbook or in their Periodic Service (P/S) Check; led to difficulty identifying a leak existed.

Narrative

I was called out to my first aircraft to remove a blue streak on the forward lavatory service panel by an FAA Inspector. I removed the dry streaking with the approved cleaner. The FAA Inspector asked me if I saw the streak. I told him I did. I told the Inspector that when I inspected the lavatory panel there was no ice or liquid present at that time. I told the inspector I had intended to return and reinspect and clean the panel but was distracted by my second aircraft check that I had forgotten. The inspector then asked to review my logbook entry and sign-off on the log sheet. He then asked for my license info and told me he might contact me at a later date. At that point our contact was over. I remained with the aircraft until it departed the gate. I was contacted by Union Representative telling me that the MD-82 aircraft returned later that day with a new blue streak at the forward lavatory panel. I believe the Periodic Service (PS) Check paperwork; and not being documented in the Logbook led to difficulty in identifying a leak. The cleaning of the streaks is not being tracked. [Suggest] documenting the cleaning of blue streaks on aircraft exteriors to identify possible leaks.