What services on macOS might leak identifiable information?
I recently began researching issues related to hardening macOS 10.12. Fortunately I found a thorough "practical guide to securing macOS" on Github. The macOS Security and Privacy Guide (a 56-Page PDF) covers multiple macOS security topics, including application level firewalls; kernel level packet filtering (pfctl); background services (managed by launchd); system daemons; DNS variables; and many, many others. 

With regard to your specific question, this "hardening guide" should be a useful resource for an evaluation of background services that might leak PII or other sensitive data. (The section on Spotlight Suggestions was certainly an eye-opener for me). Finally: the guide concludes with links to dozens of "Additional Resources" and "Related Software" involved in hardening macOS.