For those who aren't familiar with these terms, traditional cargo vessels carried their cargo in large spaces called holds. Simplified, the cargo was loaded in any manner of container it arrived in (crates, drums, barrels, sacks).
The majority of cargo is comes to the ship pre-packaged in containers that look very similar to a semi trailer without the wheels and simly lifted onto the ship. These containers can hold all sorts of cargo in their own packing (crates, barrels, boxes, sacks).
Most container ships still have some internal holds but these are limited in size and are usually used for shipboard purposes only.
What we don't know right now is whether this signature is coming from the ship or from a container, but is most likely coming from a container. Keep in mind, besides the possible sinister threat, this container could have previously been used for moving certain other materials including hospital waste.