Rose is correct, one of the Brandon boys transmitter cut out on the way south near/in Arkansas. But it just stopped working all of a sudden rather than the bird stopped working (died) and the transmitter continued to broadcast until the battery was depleted. Transmitters are notorious for dying but occasionally they do kick back on after being "off" but not often.
As for the other Brandon male, his transmitter also stopped working about a month and a bit ago now but he had been in one area in southern Texas for a couple of weeks and then it suddenly cut out. Its a common area for young peregrines at that time of the year - in fact Vesper was only a couple of hundred kms north of the Brandon chick's location at this same time last year - so sort of a staging area for at least some of our birds it seems. Searchers when out and couldn't find anything (and they should have been able to) so again, its very likely its the transmitter and not the peregrine.
I have posted a note to the Midwest coordinators about all the birds with transmitters (ours and the captive-bred ones) so if a birder should spot a bird on migration with a backpack, we should get notified hopefully fairly quickly.