Actually Shiny I have been thinking about this, just spoke to DH about it as I wanted a homebirth next time...
I had a few problems with DD growing in my womb prior to birth, and had about 3 scans leading up to my birth, the last of these checked the placenta and oxygen levels to see if there were any reasons here that meant DD was not growing.
Any concerns with my placenta would have been highlighted at this point, I am sure, as that was one of the things they were checking for - not this specifically but checking the placenta was functioning normally. There was nothing untoward presented in this last scan two days before DD was born.
So I thinking that, therefore, a scan might not be an accurate assessment.
My cord broke too, and so did the placenta as it was removed, under local [shock[. However, the consultant told me that there was no clear reason for the placenta to be retained, sometimes it happens and not an indicator of it occuring again.
I think they are just covering themselves by having you in hospital.
Its funny that sometimes you have to recall stories out loud to remember the details!