Containers aren't really a uniform thing aside from their size and stacking. Whoever commissioned/owns the container could decide to put a tracker in, but to my knowledge (which is from "hobby curiosity") it's extremely rare.
Also, the GPS trackers would stop working very quickly if the containers sink. Regular bulk shipping containers aren't waterproof so I'd expect the majority of the lost containers to be at the bottom of the ocean by now - at that point the trackers would neither receive GPS nor be able to send anything really. (cf. https://en.wikipedia.org/wiki/Communication_with_submarines )
GPS trackers are sometimes used, but GPS is receive-only, they allow you to track the location history if you have gotten the container and the tracker; but it does not imply having the ability to locate it if it's lost because the container can't send its location to you from the middle of the ocean - doing that would require essentially having a satellite radio with proper antennas, and that's not generally done. A naval transponder would allow it to be located if you're within perhaps 40 miles or something in the ballpark, but events like this tend to happen much further from a coast.
In any case, that would be worthless, in almost all cases it wouldn't be worth to explicitly go out and retrieve the container even if it would still be undamaged and floating (which is unlikely).
If they did, the containers at the bottom of the stack would be blocked from getting a GPS signal.
Last time I looked into this, which was over a decade, some of the containers at the top could have GPS and ZigBee, and would relay their position to the other container using ZigBee.
I also remember that the company I worked for, I was told that we paid a premium to have our containers placed lower on the stack, in the event something like in the article occurred.
Wouldn't they just sink ? Most of the things shipped wouldn't survive a trip in salty water I assume, and even if they do I'd I doubt we'd spend any money on a rescue mission
Most would sink yes. Ideally all containers would be ensured to sink, except for those containing materials hazardous to the marine environment, in which case they should contain floats (activated upon being thrown into the sea) and an AIS transponder.
Also, the GPS trackers would stop working very quickly if the containers sink. Regular bulk shipping containers aren't waterproof so I'd expect the majority of the lost containers to be at the bottom of the ocean by now - at that point the trackers would neither receive GPS nor be able to send anything really. (cf. https://en.wikipedia.org/wiki/Communication_with_submarines )