Field Not returned when requested in the API

Using the API (both the explored and a proper call to the REST API) to request fields in the contact module for a given contact.

The requested field does not get returned in the JSON.

The Field is contact source/source which has been word replaced with the title "bridge id."

The same approach is used with the external_identifier field which is also word replaced to be "NHS no/CHI no," however the external identifier is returned properly.

This is the only field I have noticed that exhibits this problem.

At first I thought there might be a configuration item restricting access to some fields but I cannot fine anywhere that might be doing that.

API Explorer
/test/mirth/wp-content/plugins/civicrm/civicrm/extern/rest.php?entity=Contact&action=get&api_key=userkey&key=sitekey&json=

{"sequential":1,"return":"last_name,first_name,source,external_identifier","id":29989}

Returned JSON
{
"is_error": 0,
"version": 3,
"count": 1,
"id": 29989,
"values": [

{ "contact_id": "29989", "external_identifier": "", "first_name": "wqewwe", "last_name": "7867676786786", "id": "29989" }

]
}

{ "contact_id": "29989", "first_name": "sadsasad", "last_name": "fsdffghgfgfh", "id": "29989" }