From what I can tell, the 300K number comes from an Inspector General Report in 2024.
It appears, to me, that what happened was a lot of unaccompanied minors were released to sponsors in the US without a scheduled court date. So they are living in the US but are not working their way through any kind of process to either become a permanent resident or be deported.
YMMV on how much difference you see between "missing" and "unmonitored" in this context.
"Located" also seems pretty ambiguous. I can't find much on this beyond the TV hit that Faulkner did stating Tom Homan told her as much. How are they defining it? Based on how I interpret the IG report, "located" could mean we sent a notice to appear to the address given when they were released. It could be something more than that.
It is good these kids are being located and make their way through the system. Hopefully there are other reforms to improve communication between DHS and HHS to prevent the ball dropping like this in the future.
Here is the PDF IG report: https://www.oig.dhs.gov/sites/default/files/assets/2024-08/OIG-24-46-Aug24.pdf