Well, since it's down that low, and I expect you don't have any
bottom suction fittings, that rules out all the plumbing.
You'll havta get rid of the leaves, but while it's only a foot deep,
makes it simpler. (no underwater diving)
Inspect the water line....
check all the seams for splitting....
seam leaks are harder to repair.
Walk/crawl around the bottom and look/feel for a small depression
that would be made by water leaking out...a small leak, over time,
will create a depression where sand has washed out.
More obvious spots would be where the ladder sits, etc.
If it's so small or hidden in a seam you can't find it,
the pool company has (or knows who does) an electronic gadget
that will find it.
5 years is not old for a liner, so you should be able to
repair and get more life out of it.
Just Common Sense......
err....I'm not a Doctor, but I'll take a LOOK ! !