Strange bombardment during peace

[Indiansmoke]

Usually late in the game, after airships, sometimes it seems like someone is bombarding 1 or 2 of my cities without that actually happening. You get the bombardment sound, the units unrest and can see a stelthed (or more accurately semi transparent) air unit (usually airship) flying over my cities when that happens.

There is no damage, no defence reduction, no war with anyone, just the visual and sound effects.