Did you occupy the colony at all? Because, again, I've seen those lines, and not on an alien colony.
Question...how low did the unrest need to go for that to show up? I've had mine at 80% with nothing.
My stability didn't go below 90% before I sent in the troops, at which point they kept things stable.
I am fairly certain that it is actually unnecessary to use garrison/police forces to reduce unrest because even if unrest causes political stability to drop to 1% (which seems to be the minimum) the population doesn't rebel or succeed from the empire or anything like that. I believe there is a penalty to manufacturing however.
For example, I once accidentally flooded Mars (book of Genesis style) and reduced the population capacity from about 2 billion down to about 200 million. The population was rather upset about this and I gave up trying to reduce unrest so political stability eventually dropped to 1%. However, Mars still has about 1.4 billion citizens present from the pre-flood population.