This is a reasonable concern. And indeed we only (currently) track holdings of publicly-traded securities, where for some senators a decent fraction of assets are in privately-held companies or things like property investments. We take some solace in the fact that officials are required to disclose trades by spouses + dependents (which we include in our calculations), but you're right that we could be missing more illicit activity.

We hope that this work highlights potential dangers of allowing officials to trade in individual stocks, and encourage more robust legislation that would limit the sort of activity you're describing. Obviously very idealistic, but that's the goal.