Google keeps a list of everyone you reach out to or receive an email from on Gmail. Even if you don't add that person to your contacts, you can easily find them in the "Other" category, accessible at contacts.google.com/other. However, there was no way to get to that list from the Contacts app on Android. Google has now rectified that oversight, but only in search results.
Simply start typing a name in the Contacts app and you'll get suggestions from your saved contacts first, followed by "Other contacts" from your various Google accounts. Tapping on any of them will reveal whatever details are associated with them, and you can easily reach out to them or properly save them to your address book.
• Other Contacts (from contacts.google.com/other) now appear in search results
The feature was just announced in the app's changelog, but we can see it in action on the newest Contacts v3.17.1 as well as a much older v3.15.2, leaving us to think it was either server-side enabled or it's been slowly rolling out for a while. It could also be that Google just remembered to add it to the changelog even though it's been live for a while — insert Google shrug. Regardless of when it appeared, the option seems to be live for all now, and you can get it whether you download the app from the Play Store or grab it manually from APK Mirror.