In all operating systems (including Mac OS X) the trash never actually deletes files, rather, the space that each one occupies is assigned a marker to say that this space virtually does not exist and can be occupied.
If for personal or professional security reasons we require a secure erasure on all occasions, enabling secure erase is always essential and that we will do by going to:
Finder menu> Preferences> Advanced.
There we have the safe mode box, which we will enable. Easy and simple.
Source | applesphere
It is very important to securely erase devices before disposing of them, otherwise confidential information may leak.
Still, there is always a damn file that cannot be deleted because it is being used at the time!