People are not much careful of their money, especially coins, actually. It’s common to find coins (or even banknotes) in various places. Such places are..
(In alphabetical order)
1) After a live concert
Of course during a live concert you cannot do much because you are stranded. But during concerts crowd’s attention is focused on the band/artist they paid to see. Their stuff comes second. Only God knows how many items (and money) they were lost during live concerts. Kinda makes sense actually. If they paid 50 EUR for the band, would they care for just 1 EUR they lost?
Lucky those guys who are going to clean the stadium after the live. How much money are they gonna find?
2) In coin-operated shopping carts
Well not much money there (usually 0.5, 1 or 2 EUR coins), but better than nothing, right? People might forget their coin on the cart because they might be in a hurry.
3) In ticket vending machines
Like in shopping carts, people might be in a hurry and after they get their ticket they might forget to get their change. So you might get lucky frequently in this case. But don’t expect any huge amount of money.
4) Into a club
I remember a friend of mine telling me, that he found 20 EUR dropped on the dancefloor. Makes sense. People at clubs often get wasted, which means they don’t pay attention to their stuff.
5) On a bar stand
Perhaps those coins are left from people who might took something like a coffee (takeaway). Otherwise who could forget their money, if they were on the bar stand for like half an hour?
6) On cafés’ tables
Usually those money are supposed for tipping, So instead of waiting for the waiter, they leave their tip on the table and take their leave. Wrong move, because some lucky person, might take them first.
7) On rainy streets
Obviously after rain has stopped. Why? Because when it’s raining people use to be in a haste in order to find some cover to protect themselves from raining. So they are not much careful of their stuff. But keep in mind that if a banknote fells on some wet place it won’t get destroyed. Many people ignore this fact. So if you find one but it’s wet pick it up.