It's not a field. It's a variable. The $$ at ths start of this variable name indicate that it's a global variable. Global variables retain the value set to them until the file is closed.
Take a look at your scripts and script triggers associated with this layout. There's probably a script step with Set Variable [$$Current_Invoice_ID ; in one of those scripts.
I didn't know this: "A global variable can be used in a calculation or script anywhere in a file, for example, other scripts or . The value of a global variable is not cleared until the file is closed."
Thank you very much !!