For example, I may spend $50,000 in a year. But I don't need to have $50,000 at any time to do this. Say I only trade with one other guy. I sell him potatoes and he sells me oranges. I sell him $1000 a week, he sells me $1000 a week. We only really need $2000 in this economy. But there might be $100,000 of economic activity in any year. This is because we trade 50 times the same money.
No one is going to sit on that much of their cash. They'll spend it and they'll earn it.
And that's really the simple way some economies decide how much money to print each fiscal year. They assume these 2 guys if their individual output grows at average gdp they need only that much more cash to circulate.
But yeah, we also remember not all of these are instant transactions or even recorded ones. IOUs still really prevalent where I come from, recorded only in pocket size notebooks or in memory.
I almost am sure that might qualify for underground economy, even if the intention is far from sinister!