I agree that Trump knows of threats and that is the reason for the lockdown. As to the threats, they and their sources can be of many varieties and possibly many attacks going off at about the same time on the part of coordinated actors: anthrax release in a populated area; dirty bomb; smallpox, regular bombs, multiple mass shootings, semi’s plowing into crowds, etc. All of which are harder to accomplish or cause less damage without crowds.
Now, it doesn’t mean that something still can’t happen at a grocery store. But multiple somethings at multiple grocery stores would be suspicious.
If Americans are staying still in their homes hopefully questionable movements by some people will be easier to spot.