telling fibs

How do you deal with children (3 and a half year old) who tell fibs?

I have a child who has been with me for a couple of months now. He doesn't tell malicious lies, but more like fantasy land fibs, for want of a better phrase! For example, if I say we are going somewhere, he will say that he has been there with his grandma, when I know he definitely hasn't. If someone mentions a toy he will say that he has got it at home too, or if we are trying a new food he has tried it at home etc. He will add details which gives it away as a 'fanciful fib', but the details are not always particularly pleasant. DH even commented on it today. He was a bit concerned (as am I ) that he could make up a fib about me, or his time here.

I have written a couple of examples in his daily diary. I haven't mentioned that I think they are fibs at all, just mentioned in passing that child said xyz, knowing full well that mum will know that it was a fib on his part. I just want to let her know what he is like. I think I'm going to make a point of writing down every 'fib' that he says, as I think it needs nipping in the bud.

Any tips?