CakePHP: Getting Validation errors for nonexistant fields

I am having some strange issues with a form in CakePHP. I am trying to save data that I got from a form to the model Booking. Some contact data, a message, a phone number and an email adress, standard stuff. The data is posted and I am trying to save it via

$this->Booking->save ($this->request->data);

But as I am trying to save, two fields don't pass the validation and stop the saving process: phone_buyer and carsign.

The strange thing is that these two fields are not part of the form I am trying to save. They don't appear in $this->data, as expected:

$this->request->data:

    array(
    'Booking' => array(
        'question' => 'Lieber Vermieter,
ich würde gerne im angegebenen Zeitraum auf deinem Parkplatz parken. Ist das möglich? Ich freue mich auf deine Rückmeldung.
Liebe Grüße',
        'start_date' => '30.10.2014',
        'end_date' => '30.10.2014',
        'sellerid' => '294',
        'buyerid' => '301',
        'parkid' => '10468',
        'status' => 'Angebot',
        'answer' => 'Blserntesn7x6 u fcgt c c ',
        'ppemail_seller' => '*email*',
        'phone_seller' => '0532732123',
        'acceptrules' => '1',
        'acceptrules2' => '1',
        'id' => '1012'
    )
)

...and they are not part of the form I am submitting, either. They are not even in any other form on the page. They do however exist as fields in the database and have validation rules in the model:

Booking.php (Model):

class Booking extends AppModel {

public $validate = array(

        //(...)
        'phone_seller' => array (
            'rule' => array ('notEmpty'),
            'message' => 'Damit dich der Mieter kontaktieren kann, brauchst du eine Telefonnummer.'
        ), 
        'ppemail_seller' => array (
            'rule' => array ('notEmpty'),
            'message' => 'Um das Geld der Miete zu empfangen, benötigst du ein PayPal-Konto.'
        ),

        // These two are giving me headaches:
        'carsign' => array( 
            'required' => array(   
                'rule' => array('notEmpty'), 
                'required' => false,
                'message' => 'Du brauchst ein Autokennzeichen, um den Parkplatz zu buchen.'
            ) 
        ),
        'phone_buyer' => array( 
            'required' => array( 
                'rule' => array('notEmpty'), 
                'required' => false,
                'message' => 'Damit dich der Vermieter kontaktieren kann, brauchst du eine Telefonnummer.'
            ) 
        )

);

}

But according to the documentation i have read until now, the rule notEmpty should not do anything if the corresponding field is not a part of the submitted form. To be sure, I added required=>false, but they still throw errors at me. I would be very glad for any help, thank you in advance. :) Sorry if some things are hard to understand, english is not my native language.

Update:

Thanks for the suggestions, I found the problem. I wrote it in an answer below.

I found the problem.

The issue was that in some code before the save, $this->Booking->read(); had been used to get the contents of the database. Since that function overwrites data in the model (as I learned here), the data that I was trying to save was lost and replaced by contents of the database (in which these specific fields were missing). I replaced the function with $this->Booking->findById($id); and everything worked fine.