> Why not China, for instance?

A couple reasons:

1. China's not particularly known to conduct this sort of activity this far from their mainland.

2. What would be their motive? China is actively trying to fill that "superpower" void being left in Europe by President Trump's unpredictable behavior.

> Or a random terrorist group?

Plausible.

> Speculation is fun but it's important to actually make statements grounded in reality.

I look at it from the standpoint of motive and history. See "GRU Unit 29155"[1]. Russia has both. Russia is on the brink of war with Europe.

1. https://en.wikipedia.org/wiki/GRU_Unit_29155#Activities