Recently rescued a 3yr .old cocker spaniel. He ...

he's waiting for you to take it from him and play fetch.

I think the others that gave you an answer to your question may be right. But it sounds to me like he may be missing a sibling that he was close to. You said that you rescued him maybe there was another dog at the shelter that was his sibling and they played with toys together and he thinks that if he carries it around then the other dog might come by and he will be ready to play with it. Try playing with him and the toy to see if he will play with you or if he takes off. If he takes off with it then he is missing his sibling.

Hes playful and the toy came from you

It is likely that he gets comfort from 'owning' the toy and carries it with him.

If he greets you and others with a toy in his mouth (like when you come home from work) it is his way of telling you he wants to play and he has an 'offering' for you.

My son has a Jack Russel that is never seen without a ball in her mouth and always wants someone to toss the ball so she can retrieve it, but yours is a different story. My son has another dog, a Dachsund-X that likes to cozy up with a blanket and sucks on it. He puts a big wad of it in his mouth and lies there. I think he finds it comforting. Naturally, he has his own blanket. But, if he can't find it, he will adopt another. He will even go into the laundry room and find something like a towel. Funny thing, he never damages anything.