Searching Custom Fields in Sales Invoices

I added a custom field to my invoices which includes data about my sale that I would like to search for in the future. Unfortunately it seems that custom fields do not show in the search result. Is there away to make it appear in my search.
Basically we are adding the serial number of items we sell to the custom field and I would like to be able to search for it.

Are you adding custom field on sales invoice or sales invoice line?