Holding a UK callsign, it doesn’t take much to recognise another one when you see it. This wasn’t any normal APRS station, though – it was a high altitude balloon on its way around the world!
Leo Bodnar M0XER has been making and releasing high-altitude balloons for some time, and a few have made it all the way around the world. At the time, this one (using the SSID M0XER-6) was just over 100km south of me, over the Olympic National Park in Washington state. It eventually came within 95km of me, before heading eastwards and eventually crossing the border into Canada.
Leo’s balloons are a great example of what APRS can be used for, and more information (including a map of the flight path) about this particular balloon (also known as B-66) can be found here.