2 hours later, I think I lost approx cm of water or so, probably less as my pond is not really level so one area would appear to lose more. A positive is that the pipes are not leaking at all, bone dry, which is fantastic.
Leads me to believe that the silicone is leaking, quite a lot. But yet I don't see much loss of water, though I guess when water's running it's also higher in the bog and more water would be leaking out. But then the water loss is a bit hard to see with eyes without a strong flashlight, because the water could be quite literally dripping down and it'd be hard to see because its constantly wet, I do notice that it is leaking but is that really enough to lose half a centimeter of water or even a cm?
Part of me wants to buy liner and line the barrel then connect both that way. But that would also mean removing all this gravel which is a lot, it would though give me an opportunity to set the barrel right which would be great. Just an option I'll think about. It could also be the liner near the waterfall, but this much water lost is not really possible I think with a liner leak. And I have geotextile as well so doubt it. Liner is the best option and probably not cost a lot, but adhesives etc cost a lot so I'll think about it whether to do that after I see whats going on.
Not gonna waste time anymore, tomorrow I take concrete action and dig out the area around more, remove some rocks near the waterfall and try and understand what is going on...