This is an idea I came up for my wife who is building a new guard house complex in her community. There are several sleeves under the driveway for various things. I suggested that she establish a couple reference points and shoot pictures directly over this "pin" with large signs having the measured distance away and directly over each sleeve at a couple points.
X number years later you should still have enough existing structure in the background of that line of sight to reconstruct that picture and locate your sleeves. It should only take 2 points to define any thing in a straight line but I would have 3 as a sanity check and in case a tree gets cut down or a building is gone.
Your initial reference point should be something you can easily locate, like 3' from the corner of the building in line with the long wall or whatever works best for your shots. Store these pictures with the documents you are not likely to lose, now days you could store them on a dozen different computers too.

Just a thought
Greg Fretwell