If it was a normal day I would understand that it was possible to miss it but I would have hoped they'd pay closer attention given it was his first day!
They should know how she got the bruise. At the nursery we use, we have to sign a form for any accidents and any noticed marks have to be explained and documented (even if they’ve happened at home). Have never come in to find her stuff scattered around either, as every child has their own basket. Just occasionally they need to hunt for her shoes if she’s taken them off herself.
As a nursery practitioner myself,they should have been watching all of the children at all times and they should have been able to tell you how she she bruised herself even if they never seen this happen as there should be cameras in the rooms for them to check back on