Where there are bins, Use your Dowsing Machine. It will not normally blink at the trash can, If you have left some items there, Get them Then Check the dowsing Machine. Try Looking Everywhere! Sometimes, Rare candies are in there, And most of the time, Berries. Rare candy: 10% Berries: 50% stat Enhancers: 30% MooMooMilk: 12% And more. As for The Earlier GENS there is HG where there are SOME trash cans and they MIGHT be full. I know the Percentages of Getting items: Rare Bone: 30% Rare Candy: 10% Berries: 12% Eveloutionary Stones: 50% Plates: 5%. As of EARLIER Gens (Groan) In KANTO R/G/B There ARE Trash Cans That Will be full sometimes. Percentages: Rare Candie(s) 40% Berries 80% Rare Items: 5%
This should answer Your question.
The Leftovers, Are not found in trash cans. They are found in the castelia Sewers, Near the science lab.