They didn't want to suddenly seem like a big group right after almost getting caught at the construction site. sure Rachel and Cassie are friends. Marco and Jake are friends. Jake and Cassie have a secret almost-an-item thing going on. If all five of them started hanging out together right after six kids were seen running away from the scene of Elfangor's death, it would raise suspicions. Though after things cooled down a bit, it would be more okay to start hanging out more. It wouldn't be totally out of left field, since they all have some connection to Jake.
To add to this thought a bit, I've actually always kind of wondered why the events of the first book didn't carry over into later books more than they did. The Yeerks knew that a bunch of human kids had been wandering around the construction site, right? Even if they didn't turn out to be the "Andalite bandits," the Yeerks
must have known that those kids were almost certainly witnesses to a bona fide alien encounter. So they had a very strong motive to find out who they were, infest them, and make darn sure they weren't spreading the word to their friends. Especially given the lengths they went to, in book #14, to cover up the existence of an alien artifact that nobody even knew what it was, it's a bit odd that the Yeerks would just let that whole thing drop. Chalk it up to a KASU, I suppose.
Going back to the subject at hand, I agree that they were probably being too paranoid, at least after the Yeerks stopped looking for the kids from the construction site. But at the same time it's safer to be paranoid than a controller, and it definitely added to the atmosphere of suspicion and distrust that made the books so interesting to read.
However, another thing to keep in mind is the various other, well, 'extracurricular' activities the Animorphs have been involved in. So yeah, the Yeerks may never have just been like "Hmm, those kids are hanging out a lot, that seems suspicious!" But when you start adding in the fact that they tend to skip classes together . . . and those skipped classes roughly correspond with Andalite bandit sightings . . . and then there was that time in book 12 that Rachel showed up in Chapman's office wearing those weird bearskin boots that looked a little too realistic . . . and then there was that other time in book 29 when Rachel and Jake's cousin spazzed out at the school dance . . . and suddenly it kinda
does seem suspicious that those particular kids are constantly hanging out.
Thus, maybe it's not so much about the Yeerks putting 2 and 2 together, and more about them putting 2 and 2 and 2 and 2 and 2 and 2 together. Maybe every little thing that the Animorphs could do to keep themselves a little further off the Yeerks' radar, could have done them good.
