Let $\Sigma$ be a covariance matrix. According to the material in this link,
If the elements of $\Sigma$ are all positive, most of the off-diagonal elements in $\Sigma^{-1}$ will be negative.
thisThis is actually written about the correlation matrix, but the principle should be the same.
What does "most" here mean? Is there a common condition that would make all the off-diagonal elements negative?