In case of receiving a warrant, that's exactly the part that would be missing, and they wouldn't be able to post it anymore.
So if Silent Circle doesn't update this over the next few days explicitly saying they haven't received a warrant until that date, then it's safe to assume that they have received one.
In a fully hypothetical situation.
The US government has obtained secret warrants and a mandate to ensure that the secret warrants stay secret.
The warrants are served and whatever agencies are involved start doing their penetration / searches / seizures.
SilentCircle does not update their canary as is their policy.
The US government realizes that this action has compromised their secret warrants and take steps to mitigate the situation. They do this by coercing the person or persons at SilentCircle that can post an update to post an update stating that everything is just peachy now. Followed up by another post on their blog, or here on hacker news (as happened the last time they did not update the canary) stating that it was a misunderstanding or a simple error and no to worry everything is fine.
The value of the SilentCircle has in my opinion been nil since the last incident occurred when it was not updated.
I would like to believe that our government will act transparently in accordance with democratic principles and in accordance with the public courts but this is not always the case. It may be the case in most cases.
I would also like to believe that SilentCircle employees would not agree to "fake" their canary under any circumstances, and this is probably 90% certain. If the government arrested a high level employee for not cooperating, it would certainly leak out at some point. I do not know what methods are available to secretly force an American to do the bidding of the government in a case like this but they may have means of being persuasive.
I fully agree that this is a hypothetical situation and maybe very unlikely.