Here are some examples of situations where someone might say "that sounds ominous":
* A friend says they're going to meet someone in a dark alley. "That sounds ominous! Be careful."
* You receive an anonymous letter with no return address. "This letter is really unsettling. It sounds ominous."
* The news reports a sudden increase in crime in your neighborhood. "That sounds ominous. I'm going to be extra cautious."
In essence, "ominous" implies a feeling of unease or apprehension about the future. It suggests that something bad might be about to happen, or that there are hidden dangers lurking beneath the surface.