Hello Laurent Iltis,
Since you are using load-on-demand the child nodes are not loaded until the parent node is expanded. Thus the treeview does not know initially which are the child nodes and cannot find the node you are searching for.
One solution would be to search for the given string in the database and if you find a record, get all the parent records and create the respective nodes manually.
Regards,
Veselin Vasilev
the Telerik team