[It's not so much a sign as it is a tapping, a scratching on the wall.
A message goes out along old Soviet wires to transmission stations that send out even older coded pulses. They are as dark and as quiet as such messages can be. They are all but forgotten, monitored now by only a few, highly specialized intelligence agencies. SHIELD, for instance, might be listening. This particular message comes from what seems to be a failsafe switch, to be triggered only in the most dire of circumstances.
( Cut for length. )As for who set off this chain reaction, who pressed a button dormant and forgotten for forty years, there is no sign of forced entry in the hidden room in Ukraine. And not so much as a strand of hair.]