They won't have migrated. I too have a robin and I didn't see him/her for a couple of days and was starting to get concerned when he suddenly came back. He (or she) flies off with a load of food in its beak, so young are obviously being fed somewhere. I had a lovely little pied wagtail that was quite cheeky, but he vanished about seven weeks ago and I've never seen him since. I'd be interested too if anyone on here knew why the birds disappear. I'm hoping it's not a cat.