Insiders could also buy shares in order to hide something, but then the amount would probably be a giveaway.
I was thinking about all this after reading George Muzea,who used insider transactions as an indicator. He also used the remarks of pundits in the media as a contrarian indicator, calling all the noise “the trivial many.” Even before reading him, I would often think that I should do the opposite of whatever Jim Cramer recommends.