Search a list of Dicts using a list of items and find if the item exists under the attribute 'value'. Store only the Items NOT found as a new list

This one has been stomping me for a good while now. Hoping you guys can help.

I have a list of dicts registered like the example below (These are 'objects' from a Firewall)

"objects": [
      { 
         "name": "Test1",  
         "type": "ip-netmask", 
         "value": "8.8.8.8"
      },
      { 
         "name": "Test2", 
         "type": "ip-netmask", 
         "value": "8.8.4.4"
      }
   ]
}

Using a list of IP addresses called 'sources' I am currently able to search the 'values' in 'objects' to see if they exist. If they are found, I create a new list using set_fact and populate this with the 'name' of the object associated with the found 'value'. This is working OK.

The problem I am trying to solve is how I can create a new list with the 'sources' that are NOT found in my search of the values.

My working code for finding existing 'sources' and storing as names is as below:

vars
sources: ['8.8.8.8','8.8.4.4']

tasks:

  - name: Fetch all objects and store result
     panos_object_facts:
      provider: "{{ cli }}"
      device_group: DG_Test
      name_regex: '.*'
      object_type: 'address'
      register: result 

 - name: Search result for our sources and store as list if found
     set_fact:
        existing_source_addr: "{{  existing_source_addr|default([]) + [(result.objects | selectattr('value', 'search', item ) | list  | first).name ] }}"   
      with_items: "{{ sources }}"
    - debug: var=existing_source_addr

This returns an example as below:

"existing_source_addr": "['Test1', 'Test2']"

The below example is the code I am testing to create a new list which only contains the 'sources' which are NOT found. This is not working as expected.

vars
sources: ['8.8.8.8','8.8.4.4','4.4.4.4'] 

# 4.4.4.4 does not exist in our list of dicts 'objects' #

tasks:

  - name: Fetch all objects and store result
     panos_object_facts:
      provider: "{{ cli }}"
      device_group: DG_Test
      name_regex: '.*'
      object_type: 'address'
      register: result 

  - name: Search result for our sources and store as list if NOT found 
      set_fact:
        non_existing_source_addr: "{{ non_existing_source_addr|default([]) + [item] }}"
      with_items: "{{ sources }}"
      when: item not in result.objects | selectattr('value', 'search', item ) | list
   - debug: var=non_existing_source_addr

This returns an example as below:

"non_existing_source_addr": "[u'8.8.8.8', u'8.8.4.4', AnsibleUndefined]"

The condition is true as the 3rd item is not found as expected, but my variable is being set with all items from the list 'sources' + 'AnsibleUndefined' for the entry which does not exist.

Is there a way to have this new list ONLY contain the item which was not found '4.4.4.4'? This would allow me to then use the new list to create the missing objects.