How can I mark capped drip line outlets from irrigation systems?
Tie a piece of red or blue polyester twine onto the end of the pipe(s) and let it lay out on the ground surface for a few feet. Even if it gets covered over with some mulch a little bit of raking should quickly reveal its location.

An alternative is to make a carefully measured out map of that area of your yard. Put that into your computer for storage and also print out a copy which you would store along with your other important papers.