Wondering if anyone can give me any clue as to what to expect to see in or around the pool when it develops a leak.

Would water literally just leak into the surrounding soil, or is there some smart plumbing that is routinely installed with pools which prevents uncontrolled leaking into the surrounding soil and perhaps channels it into some kind of disposal water pipes and into the stormwater system??

Normally just leaks out.

Mates pool is leaking at the minute and the pool liner is floating on the water that can get out (very much clay soil/bad drainage)

Hello, snorkles and some food dye, if you spot cracks within the pool or spa just drip a few drops of food dye along the cracks to see if the water is escaping or not, cheers.

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Fill a bucket with water beside your pool. Measure the drop in water level of the bucket and that of the pool. After a week they should both have dropped by the same amount through evaporation.

If the drop in pool level is greater you have a leak

the test sequence above is ok but is easier to place bucket on a step & fill to same level as pool . it is easier to see any difference in levels.
is pool concrete , fibreglass or liner. if fibreglass it will have a stand pipe fitted at the deep end . see if there is any water at bottom of pipe. if there is test it for traces of chlorine if there is you have a leak. use food colour test around hydrostatic valve first then fittings & lights if nothing there check any areas around pool that look like a possible crack.
cheers andrew

We had a leak, and could tell by the water level dropping. We did the bucket test too, that's a great idea and gives you an idea on how quickly you are losing the water.

Turned out we had a hole which they pool installers came out and plugged up with aqua putty and colour matched it to the pool. this was just over a year ago and it's still all in place.

We had to be quite forceful to the pool company that we had a leak, they said it was "impossible"