I've just found out what it is: a current xml namespace which gets magically injected in the following way:
```
scala> q"""<x><y/></x>"""
res4: reflect.runtime.universe.Tree =
new _root_.scala.xml.Elem(null, "x", _root_.scala.xml.Null, $scope, false, ({
  val $buf = new _root_.scala.xml.NodeBuffer();
  $buf.$amp$plus(new _root_.scala.xml.Elem(null, "y", _root_.scala.xml.Null, $scope, true));
  $buf
}: _*))

scala> q"""<x xmlns:edi="foobar"><y/></x>"""
res5: reflect.runtime.universe.Tree =
{
  var $tmpscope: _root_.scala.xml.NamespaceBinding = $scope;
  $tmpscope = new _root_.scala.xml.NamespaceBinding("edi", "foobar", $tmpscope);
  {
    val $scope: _root_.scala.xml.NamespaceBinding = $tmpscope;
    new _root_.scala.xml.Elem(null, "x", _root_.scala.xml.Null, $scope, false, ({
      val $buf = new _root_.scala.xml.NodeBuffer();
      $buf.$amp$plus(new _root_.scala.xml.Elem(null, "y", _root_.scala.xml.Null, $scope, true));
      $buf
    }: _*))
  }
}
```