I think you are right, in that anybody's "hinky meter" would eventually go off. But... the couple was not only anti government , but also involved in a Mormon fringe group(s).
Mormon fringe groups are usually polygamous and despite their self image as self reliant pioneers, also have a long history of welfare fraud. Thus, there are needs to hide children regarding polygamy centered custody disputes, or conversely, shuffle children around to support welfare filings in multiple jurisdictions.
Then factor in that it is common for Mormon fringe groups to maintain isolated compounds or communities. Their fringe group was apparently not part of the polygamous compounds in US, MX or Canada.
But, the concept of having access to remote compounds is not unusual for the sub culture. Likewise, some people may have ties to more than one fringe group, and by extension, have access to different isolate communities.
So....
The couple's associates live in a world where "oppressive" government agencies are everywhere and constantly plot against families "In the know". They also live in a world where children frequently disappear only to re-appear later and a world where entire families escape into compounds for various reasons - then resurface no worse for wear.
Thus, hinky meters were very slow to activate.
