It's civilian and... I don't think there's a rule for that.
Most balloons don't fly that far. For the WW2 Japanese case, which was designed for the purpose, only single digit percentage arrived in the other continent.
Well, that’s very doubtful. It’s a big and sophisticated balloon, and likely expensive, not an amateur one. Ww2 was 70 years ago, tracking has gone up by leaps and bounds since then.
But assuming for a second that they did lose it, the only responsible action would be to notify the authority, in China and other countries on its potential trajectory. Such a big object can be hazardous to air traffic if left untracked. It’s very hard to believe some innocent party just sent it up, lost it, didn’t tell anyone, not even its own country’s authority. Malicious intent is lot more plausible. Especially if the 2nd balloon over South America is confirmed.
But assuming for a second that they did lose it, the only responsible action would be to notify the authority, in China and other countries on its potential trajectory. Such a big object can be hazardous to air traffic if left untracked. It’s very hard to believe some innocent party just sent it up, lost it, didn’t tell anyone, not even its own country’s authority. Malicious intent is lot more plausible. Especially if the 2nd balloon over South America is confirmed.