Board Thread:Theories and Speculation/@comment-44807718-20200106051049/@comment-1210805-20200106074836

It has been shown all through the series that off-duty shinobi lower their guards within their own village. Even Tobirama was shown to stop using his sensory abilities in the village, even when having an important high level discussion with Hashirama. The only times we see the Hyuga with their Byakugan active within their compound is when they're training.

In regards to the night of the incident, Hiashi was pretty quick to react, so it's likely they were ready for such an attempt. We saw the diplomatic problems that resulted from him dealing with the intruder after he had taken his daughter, just imagine how much worse it would have been if he attacked before the Kumo-nin actually did anything.