There truly are particular types of behavior, body language & "Welcome Flags," that most people can easily pick up on. Depending on the environment and situation at the moment, we find we are in a room full of strangers.....I know that I may automatically "look around" a bit and scan the crowd without staring.
If I appear relaxed and smile, invariably someone also looking for a friendly, relaxed person, will approach me or welcome my approach. I feel it's much like a silent mutual invitation to interact. Similar personalities seem to find one another and can more or less read the signs and feel the vibes.
I believe that often it is so obvious, it's like wearing a sign.
Certainly the opposite would hold true as well. Introverted/shy people or someone who avoids eye contact, does not smile or seem comfortable, would not be an easy person to approach
I see your question as going hand in hand with "First Impressions." (keeping in mind that 1st impressions are accurate about 90% of the time).......Great Question, midget.