It seems to me that in '1', 'unnoticed by her' should be modifying a noun and not the entire clause 'he slipped the coin into his pocket'.
Hello, Navi—I agree with your observation and do not see (1) as equivalent in meaning to (2). I understand (1) to mean that he was not noticed by her when he slipped the coin into his pocket. Here's a sentence that you can use instead of (2):
3) He slipped the coin into his pocket, unbeknownst to her.