Drupal 7 field forced into value on save

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Drupal

So I'm working on a project that has been through many freelancers before it got to me.

I've got this issue if I've got a sale with a field that has a field called "sale type" set to "customer sale" it will automatically set the field "authorised" to "authorised".

So to clarify the logic for this:
- Node bundle is "sale"
- "field_sale_type" is "customer sale"
- On node_save can be (insert or update)
- Set "field_auth_code" to authorised

Any thoughts would be appreciated

What I have tried:

I know this only happens after node_save hooks because if i use field_attach_update it doesn't get auto authorised.

I've also searched the entire code base from the base site through all the custom modules for the machine name of the "authorised" field which has the machine name"field_auth_code". Nowhere is it programmatically setting "field_auth_code" to 1 in an undesirable fashion. So I can only assume that it might a drupal setting or module setting that gives you the functionality on the front end to do this but i can't seem to find any menu that leads to this option.