A creature with the Invisible condition is automatically Hidden to any creature relying on sight as their only precise sense. Precise senses other than sight ignore the Invisible condition.
If you're already Observing a creature when they become Invisible, they start out as Concealed, since you know where they were.
↳ You can Seek to attempt to figure out an Invisible creature's location, though they may never become Observed in this way.
Some effects might partially foil invisibility. For instance, if you were tracking an invisible creature's footprints through snow or sand, the footprints would make them Concealed.