Yes it's only there in the morning, not throughout the day.
We feed them at night their meat and veg and they have a big bowl of dry food always down, which they nibble from throughout the day. I don't see whoever it is doing it, i think it's happening overnight or early morning. Maybe I should try giving them meat morning and night. Could they be eating something that is making them sick, they don't vomit up their food though, it does look like bile! Maybe one of the dogs is eating the bulk of dinner and someone is not getting enough. But they still always have dry food they can eat.