Jack has one from time to time. He also seems to pretend that I am there when I am not. Like one day when I was at work DH, MIL and FIL took the boys to the railway museum. That night Jack was telling me about it and I said I was sad I couldn't go too. He replied "you were there mummy".

I wouldn't worry too much. It sounds normal to me. And if it is to cope with some stress then I think it's a good sign that he is able to find a way to cope in a positive way instead of being sad and/or angry. I would say he's advanced for his age and is doing just fine.