It was in an aviary but I'm quite sure its an European species because the Center I photographed it is a Rehab-center for native bird althrough they also take care for confisticated foreign birds. However because sandpipers and other shorebirds are rarely kept by private persons - and therefor rarely confisticated - I'm pretty sure it was found in the wild in the Netherlands.
I would also go with a winter plumage Dunlin on the principle of the following ID comparisons of the beaks as both birds can be/look remarkably alike in photographs.
Dunlin -
In its winter plumage, the dunlin is grey above and white underneath and has a DECURVED bill,
Red knot -
Silvery grey on top with a white chest and small grey/greenish legs with a STRAIGHT BILL.
Quite right. I should not have tried to answer after a 5 hour drive. I knew it was a dunlin as I was typing, because the dunlin is the smallest common wader in Europe. I apologise