Well, the large BCI I had for many years would (rarely) hiss very loud- open mouth- when disturbed (when I needed/wanted to take her out). She too was bluffing, in that all I had to do was stay out of her face & simply pet her coils while she appeared to remember she knew me- & she never once turned to bite me.
After very few minutes at most, she'd stop hissing & I'd just pick her up with no difficulty at all- she was right back to being cuddly & fully comfortable being held. I think sometimes it's just that their minds are elsewhere- they're not thinking about being "pets"- they just react as the wild souls they truly are, until we "remind them" that they know us & they're safe with us. To me it appears they know us by our touch to a great extent. Some snakes go by our scent too.