They are treated like any other evidence
A chain of custody is maintained and they are stored in secure evidence lockers, this may be ant a local police station or an centralised facility, until they are no longer needed. That is, until the case they are associated with is finally resolved.
For washed-up drugs like you describe, these will likely need to be stored indefinitely as they are part of an ongoing case for which no one has been, or is likely to be, charged. There are likely procedures that allow the disposal of “cold case” evidence eventually.
Once no longer needed, non-illicit evidence (e.g. a car) is returned to the owner, illicit evidence (e.g. cocaine) is destroyed. Destruction is done in the manner appropriate for the evidence.
Illicit drugs are incinerated by private contractors who incinerate other pharmaceuticals. They have their own procedures to ensure that what they receive is destroyed. They are highly regulated and subject to audits and inspections.
Is there leakage? Probably, but probably not a lot.