Locations API uses OS Places API under the hood. It makes requests to OS Places API whenever it needs to add or update the results for a given postcode.
Each of the three environments Locations API runs in - Integration, Staging and Production - is set up as its own 'project' in the OS Data Hub. Each project defines its own set of API keys.
These keys are stored in govuk-secrets under puppet_aws/hieradata/apps, in their respective environment.
You can log into the OS Data Hub to view usage and to regenerate API keys if needed.
The credentials for the account are stored in govuk-secrets, under govuk-secrets/pass/2ndline/ordnance-survey/os-data-hub.gpg.