A Line Mechanic reports a flight crew found a #2 engine oil cap and service door not secured on a B737-800 aircraft during their walk-around; after he had serviced the engine oil.

Narrative

Received phone call that oil cap was [found] left-off after [I left my] shift. It was reported by flight crew and entered into Logbook. I got a call indicating that the flight crew put a Log entry into Logbook that the cap and [service] door were not secured. This happened on the #2 engine. Only thing I can think of is that the oil service door will close on it's own due to design. While servicing the #2 engine it is necessary to hold the service door open while servicing. After servicing the engine with the last quart of oil; (took two quarts); I put the empty oil can back on the truck. With no visual trigger to secure door and cap; I must have assumed that I accomplished that task already. Secure oil caps are a very important item for a safe aircraft. I am aware of that. I even so much as made a second attempt to make sure the door was secured before leaving the aircraft by driving out of the way before going back to the Shop; to do a second visual of the door (after servicing #2 engine; drove truck to bottom of Jetway stairs; when Logbook was completed; drove back around to the #2 engine for visual check which is in opposite direction of Shop; I remember distinctly doing this; I have a paranoia of oil caps). It appeared to be secured. Evidently it wasn't (visual performed from inside truck). I assume a Technician went out and secured the [service] door and cap. I was really surprised that this event happened. I have been at this job for over 21 years and have never had an oil cap event. It is something I try to give due diligence; to make [sure] something like does not happen. I have been on the Line 20 of the 21 years. I do Pre-departure Checks just about every day. I have done countless oil servicing on these aircraft over the years. This event has scared me. I thought I was doing a good job. It only goes to show. My solution to my problem is to make sure I leave myself a visual clue that the cap and door on either motor needs to be secured. In the past I have used a rag to secure the # 2 engine oil servicing [door is] propped open while servicing it. I am going to have to do this again as to have a visual clue; as in [Landing] Gear pins with the red flag. I might even so much as design something that will attach to the door; to prop it open; until cap is secured before removing it.