Well, probably one of the first person to note and describe that was Galileo. As he beautifully describes in his Dialogue Concerning the Two Chief World Systems:
Shut yourself up with some friend in the main cabin below decks on
some large ship, and have with you there some flies, butterflies, and
other small flying animals.
With the ship standing still, observe carefully how the little animals
fly with equal speed to all sides of the cabin. When you have
observed all these things carefully (though doubtless when the ship is
standing still everything must happen in this way), have the ship
proceed with any speed you like, so long as the motion is uniform and
not fluctuating this way and that. You will discover not the least
change in all the effects named, nor could you tell from any of them
whether the ship was moving or standing still.
Finally the butterflies and flies will continue their flights
indifferently toward every side, nor will it ever happen that they are
concentrated toward the stern, as if tired out from keeping up with
the course of the ship, from which they will have been separated
during long intervals by keeping themselves in the air.
This observation lead Galileo to enunciate that the laws of mechanics are invariant under a change of inertial frames. Nowadays we believe that not only the laws of mechanics but also the laws of physics shall be invariant under the change of any frame of reference. This is a fundamental principle of physics, called the Relativity Principle.
Restricting ourselves to mechanics, it means that there is no mechanical experiment capable of detecting the absolute motion of an inertial frame. This is actually contained in the first Newton's law (which is due to Galileo). When the fly rests on the car it has $120\, km/h$ forwards with respect to the ground. When it jumps upwards, the first law asserts that it must keep this $120\, km/h$ forwards since there is no force pushing or pulling it in the horizontal direction. It can fly forward at $5\, m/s$ respective to the car exactly in the same way it would fly respective to the ground if it was outside the car.