First contribution:

Yesterday on AC002 NRT-YYZ, starting at about FL150 (15000 feet and climbing), I spotted another aircraft, similar livery to AC (white-ish with a shape on the tail, no other stripes or blocks of colour), cruising above and far to the right of us, say 31000 feet or so.

As we climbed, we appeared to be heading horizontally parallel to the other plane, and staying safely separated by 5 miles or so. Much further and I wouldn't have been able to even guess colours. The other plane stayed almost exactly beside us (still above as we climbed) for a good 15 minutes, but once we were closer vertically our flightpaths began to converge ever so slightly. I didn't think anything of it until the other plane was moving faster toward us (again, almost a parallel path, but clearly growing in size).

It eventually ended up almost above us -- still 1000 feet above at least, but only 200-500 feet horizontally. It sat almost above us (clearly visible to me from my window by leaning) for a couple minutes, then angled back to its old course. A few seconds later, we angled gently away to the left, building a legal separation quite quickly. The other plane remained off us on the right for maybe another half hour, but after that I lost sight of it.

Odd! The pilots hadn't left the cockpit when I deplaned so I missed the opportunity to ask about it unfortunately. Maybe the northern Pacific is boring and planes sometimes get together for a chat?