UPDATE 11/7/13, 1:50 pm: Thanks to MacK for providing a link to Eckert's motion for partial summary judgment in the comments below. That motion cites much of the search warrant affidavit, but Ken White at Popehat has now posted and analyzed the affidavit itself.
In the affidavit, Officer Chavez does allege that Eckert's posture was "erect" and that he "kept his legs together" — but also that: Eckert avoided making eye contact with the officer as he asked him for his license and registration; that Eckert's left hand was shaking as he handed Chavez the documents; that Eckert "stated that he had a problem with [Chavez] searching his person"; that the canine LEO, upon being walked around the outside of the vehicle, "alerted to the driver's side of the vehicle"; that the canine LEO, upon making entry into the cab of the vehicle "a short time later," alerted to the "driver's side seat"; and that the K-9 officer said that he "had dealt with Mr. Eckert on a previous case and stated that Mr. Eckert was known to insert drugs into his anal cavity and had been caught in Hidalgo County with drugs in his anal cavity."
UPDATE 11/7/13, 6:15 am: As I implied in the OP (which is now below the fold), and as I stated explicitly in my comment at 7:26 pm last night, "what primarily struck me as remarkable about the Eckert case is that the apparent bare (no pun intended) fact of buttock clenching could constitute probable cause of drug possession." Unfortunately, as I had acknowledged by 10:14 pm, after I had finally gotten around to reading the complaint,
it seems that, contrary to what I complained about above, and how the media has portrayed the case, there were, in fact, additional alleged bases for probable cause beyond the infamous clenched buttocks -- namely, a canine LEO that alerted to Eckert's car seat and one officer who claimed that Eckert was known to conceal drugs in his anal cavity.
To be sure, as I also noted in that 10:14 pm comment, "the complaint alleges that there were problems with the canine LEO known to the officers and that the officer who made allegations about Eckert's habit of drug concealment knowingly lied about this." And to be sure, even if there was probable cause for an initial one, or even two or three, searches, that doesn't resolve questions about the appropriateness of either the officers' or the doctors' roles in performing eight invasive-but-fruitless searches over the course of several hours. Still, I was initially moved to blog about this case primarily because of the absurdity of concluding probable cause from buttock-clenching-without-more. I maintain that that would indeed be absurd. Yet even the plaintiff's own complaint presents a much more complex (if not necessarily benign) picture of the alleged basis for the searches.
I wrote the OP entirely on the basis of the two media/blog reports that I linked to in the OP, both of which framed this as the Buttocks Clenching Case, and neither of which included a link to the complaint (both sources have since updated their posts to embed the complaint or include a link to it). A successful Google search for a publicly accessible copy of the complaint, and the resulting link to it, was a last-minute addition to the OP as I rushed out the door. I did glance at the first few pages, where I noticed that the suit covers an earlier traffic stop as well, something that I hadn't seen mentioned in the media reports on which I was relying. That fact alone should have alerted me to the likelihood that there was more to the story—even from the plaintiff's own perspective, as articulated in the complaint—than the media were reporting, and persuaded me to hold off on pressing the publish button.
As he promised in the comments here, Orin Kerr has offered early this morning a typically thoughtful analysis of the case over at VC. Read the whole thing, but for present purposes, this is worth highlighting:
Some media coverage on this case focuses on the line in the affidavit that the officer had apparently said that Eckert had a stiff posture and kept his legs together, which some blog posts are reporting as the sole basis for the warrant. The HuffPost article on the case offers this inflammatory opening line: “A New Mexico man is alleging abuse after authorities conducted three enemas, a colonoscopy, an X-ray and several cavity searches on him simply because he appeared to clench his buttocks.” As a result, a lot of folks in the blogosphere seems to be thinking of this as the “clenched buttocks” case (thus the title of this post, so people know which case I have in mind). But that seems like at most a very small picture of the alleged cause, and we don’t even know if that was in the affidavit. The likely basis for the alleged cause is mostly the drug-sniffing dog’s alert to the driver’s seat that is the likely basis for probable cause.
Orin ends his post by noting that although "some readers are going to be irate that I am providing a legal analysis on an emotional case, and others will be annoyed that I haven’t reached a clear answer," his "comparative advantage is analysis rather than emotion." I actually like to think that that is more often than not my comparative advantage as well, and I regret that it wasn't this time. Mea culpa.