Keep in mind that not only US or Canada launches weather balloons. These things are well capable of flying huge distances, including across the oceans. There has been one launched by amateur radio operators a few years ago that has circumnavigated the globe - and that's not at all a rare feat.

See e.g.:

https://qrp-labs.com/circumnavigators.html

So no need to go all conspirationalist about "unmanned drones operated by foreign governments" just yet. Especially given the distances and altitudes these things were found flying at - a heavier than air device would need a ton of fuel and energy to fly that high and that far, so a "drone" that at the same time doesn't look like an aircraft is rather unlikely.