An implicit message within a message is technically referred to as:

An implicit message carried within a message is technically called sub-text.

Hidden message (A) is a loose term, not the technical label for embedded meaning.

Intrinsic message (B) is not a recognised communication term for implied content.

Non-expressive message (C) suggests an absence of expression, which does not fit implied meaning.

Sub-text is the underlying meaning that lies beneath the literal, surface message.

It contrasts with the manifest text, the explicit content stated directly.