This guy calls out to me when i'm on my own in his car and/or from his window and he rings me up asking me "where are you going?... and why are you going there?" but when other people are around he goes back to his usual self and doesn't do these things. why does he do this? why does he want to know where i am going and calls my name out to me? he isn't agressive when he says this or when he calls out to me but his voice is low (if that makes sense) i don't mind him doing this it's just that i don't understand what he is doing.

I can only give a brief response to this: He's either a nosy get or he is interested in getting to know you a bit more.  The reason he doesn't do it in front of other people is confidence.  He is confident whilst you are on your own.  When other people are around there is the risk that the other people will keep you occupied and you will ignore him.  There is also a risk that the other people will find it rude or slightly desperate or unsavoury to call out like this.  It's also more difficult to engage someone when they are with other people, particularly if his voice is low, you might not be able to hear him properly over the other people.  These people would then try to persuade you that he is a bit of a tosser and for those reasons he will avoid doing it when other people are present.  

The low voice is so that he doesn't sound aggressive, aggressive may creep you out and he will look like a guy on a building site wolf whistling at a woman as she is walking past.