The previous owner of my house installed drip lines all of their gardens. In one garden we tarped, mulched, and planted succulents. We don't want that much water in that bed, so I removed the drip lines and capped off the pipes. I realize if I want to access them later for whatever reason I'm going to have to rip up almost all the flowerbed to find them, so before I forget, I want to mark them. What is a good way to do that which will survive the elements, but won't be as ugly as say, the road cone I have out there now.
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.