I don't think you understand these containerised missiles at all. They have NOTHING to do with ports. The entire goods transport system uses containers. They travel on ships and TRAINS and TRUCKS to every corner of your country. If containerised missiles did enter the USA they could be anywhere. They could be in an abandoned warehouse in the wrong part of town. They could be in a factory that has been reduced to 5% of its historic capacity, They could be on the top floor of the parking garage where you park your car every day when you commute to work.
ANYWHERE!!!
The port danger has already been dealt with by military intelligence. New containerised missiles are unlikely to enter the USA at this time. It was possible to fix this problem wo quietly BECAUSE IT WAS A MILITARY THREAT. This enabled the use of special forces in secret, who have a licence to kill in secret when that is needed to protect national security.
See how that works?