Which conditions prefer accessing a shared variable over accepting a parameter?

When building classes, my rule of thumb is to always pass parameters, unless the caller has no access to them. For example, I always prefer this:

class Foo(FooData data)
{
  private FooData _data;
  public Foo(FooData data) { _data = data; }

  pubic string GetSomeValue()
  {
    var result = ProcessData(_data);
    return result;
  }

  private static string ProcessData(FooData data)
  {
    // do something with data and return the result
  }
}

Over this:

class Foo(FooData data)
{
  private FooData _data;
  public Foo(FooData data) { _data = data; }

  pubic string GetSomeValue()
  {
    var result = ProcessData();
    return result;
  }

  private string ProcessData()
  {
    // do something with _data and return the result
  }
}

I have come across the second example many times in the wild have never identified a good reason. Under what circumstances would accessing instance variables directly (as shown in the second example) be preferred? Or is this simply a matter of personal preference?

One benefit I have seen is shorter signatures, but a long signature is probably a smell of other design issues and therefore this is just masking it.