Does Marketo reuse or recycle old deleted lead IDs

Does Marketo re-use or recycle old deleted lead IDs? 

After a lead is fully deleted, it would be very rare for the lead ID to be re-used. The vast majority of the time lead IDs are unique and not re-used.

If you have an external database or reporting system that relies on a unique lead ID, the best practice would be to ensure these systems are up-to-date on all lead deletions.