A friend is idle for 8 hours, tried to leave IM and the IM screen says XX signs back in but becomes idle again

I asked my friend about it and he said he did not touch the computer and that he is away for the whole day. What does it mean if it says in the IM screen "XXnameXX signs back in" after being idle? Then, later he becomes idle again. What causes this? Are there other causes other than the user actually touching the computer e.g. some internet activity or receiving IMs from others?