google account - Where exactly does Android get its contact suggestions from?

I have a phone right here (Evo 3D), it has no service (except my wifi) and I've only added an obligatory gmail account which has one contact.

No Facebook, no Google+, no Twitter, etc.

Yet I got a "matched contact suggestion" notification. What is that? Where did it come from?

My main phone (TMO G2) with all of the services never gets these things and I'm not familiar with that at all.

Insight appreciated.