Well now that you have explained that you are sharing rented accommodation, with one trash deposit, that everyone shares, your question makes more sense.
You should reasonably be able to expect that no other person should go rooting through the trash for items that you have discarded, (notwithstanding that you shouldn't have discarded such items in the first place). The worrying thing about all this is, that a person has removed these items and kept them in their own personal file box, to what purpose is open to suspicion. Particularly, as your question implies that you had dropped the envelopes containing the bank statements into the trash unopened. The person concerned had no way of knowing whether you had dropped them in the trash accidentally. To try to cut a long story short, it is definitely an offence to open, read, and keep someone else's mail. It might be worth talking this through with a solicitor, most of whom will offer a half-hour consultation for free. And the last bit of advice - be more careful in future. Good Luck.