Forget fiatleak already. To my knowledge, they simply count buy order as "fiat in" and sell orders as "fiat out", which is hopelessly naive of course.
What /would/ indeed give a good idea of "fiat in/out" would be if they were able to show the fiat reserves of the exchanges and their changes... for obvious reasons, that's not happening though.
Worse, since each buy has a matching sell, they count all orders as "fiat in".If exchanges published a daily balance sheet of how much they had in each currency, we'd have useful data. Mt. Gox never did that, so it sort of became standard not to do that. (As it turns out, Mt. Gox didn't even have a daily balance sheet for their own internal use. Their internal accounting was so bad their own staff didn't notice money was being stolen. Probably because Karpeles wanted it that way.)
You sure of that? Last time I looked into it, it seemed to be working on the rule I described.
And of course, every 'buy' has a matching 'sell', but for every matched order you can say which one was the one that triggered the match. So at the very least, they should do it like that, so that a large "dump" (i.e. big market sell) counts as fiat out. It would be a bit like a crude Chaikin Money Flow...
But why am I even doing this... this site is entirely useless, and I doubt they have any intention of improving it