I've never seen that done in a Greek Church that I've gone to. It certainly sounds like a Western influence. When it's done over a body part, such as when I have a headache or stomach ache, then that's fine. When it's done over a child's head, then that is the mother's blessing. The problem is when a person does it over themselves as Icxypion described. Over the heart is the Western influence of the sacred heart worship and unfortunately (which I cannot explain), Greeks do this. I have not seen this with the Russians. But, there are Western influences everywhere. People are not educated in Orthodox spirituality.
The papists do cross themselves on forehead, lips and chest, but with their thumb. Maybe the Greeks you see are taking part of that Western papist tradition. Then they (papists) kiss their thumb. I never understood that.