It was shown in TCW which leads into ROTS. A Force bond doesn't mean you can just read minds. A Force bond means that you can be aware of the Living Force within another Jedi. Dooku can sense that Yoda is on Moraband and through that, Palpatine could exploit their Force bond to attack him with Sith Magic. He could not sense his shift towards the dark side, because he didn't try to sense his thoughts and feelings before then. He had no reason to do so. Dooku was better at hiding his feelings. Obi-wan could sense Anakin's tense feelings, but not the root cause of those concerns. Nor could he sense him turn to the dark side when he did. No, like the bond between a teacher and student that happens at one stage and you come back to visit them when you're older. You don't need a guidance councilor for that. Yoda says it to both Chewbacca and Tarfful, which he does not because of a Force bond, but because he is friends with them. Yoda has no clue what will happen. He says what he says because he will miss them. It had probably been some time since he was last on Kashyyyk and it may be some time before he can come back, if he comes back at all.