Samehada was able to pin-point Hachibi becomes it was hungry for chakra, like any hungry savage beast. Samehada might be smarter then your average shark, but no smarter then a dog. Dogs can lead humans to places, but what they can't do is tell the owner what is before him, Samehada might be expressing fear and trying to indicate that their is a threat before them, but he can't explicitly tell Kisame that C4 is there so Kisame will have to guess that bit. And even still, if someone walks up to you with a knife, the dog won't always point it out even if it sees the knife, same thing with Samehada, it might not recognize C4 as a threat, it sees Jutsu on the daily and there is no reason to believe that C4 is something that its master has not countered before (from its point of view of course).
But you're not getting my point here, let me break this down:
- Deidara creates the huge clay sculpture, Kisame decides to fall back. [A natural reaction when you see a huge creature made out of explosive clay in front of you]
- The clay sculpture erupts, with no visible huge explosion.
- Kisame will note the fact that while the sculpture seemingly had no effect, Deidara is still keeping his distance, while at the same time, Samehada shows a sign of panic at the cloud of Chakra that she senses is coming their way.
- Kisame, being the intelligent, experienced ninja that he is, decides to either retreat further, merge with Samehada, or better yet, both.
As for the dog metaphor, you're taking that too far. The sword is sentient. It has shown the ability to comprehend orders and follow them, and it has displayed feelings, along with the ability to communicate these feelings to others. Why on earth would the sword just stay silent when it feels a huge cloud of hostile, unfamiliar Chakra coming its way, while its owner, who it has known and battled alongside for years, doesn't seem to be aware of this?
Your dog example is far off for one simple reason; Samehada's entire being revolves around Chakra. Through its sensory powers, it can determine who is, and who isn't, a worthy wielder, and through its abilities [Sensing Chakra, absorbing Chakra], it can aid its master, creating a beneficial relationship for both partners. Just because a dog won't necessarily recognize a knife as a threat, you argue that Samehada will simply stay put when faced against a technique which revolves around the opponent distributing his Chakra wide across the battlefield, whereas Kisame won't be able to sense it himself? Frankly, I don't see it.