Yes it is normal - IF you are just measuring it with a volt meter. A volt meter is high resistance so no current flows. Without grounding that second terminal you would see battery voltage. It's like measuring water pressure with the valve closed.

(from Virginia.edu/wikipedia) While some modern car horns are actually specialized computer audio systems, the old-fashioned electromagnetic car horns are still common. An electromagnetic horn uses an electromagnet to attract a steel diaphragm and turns that electromagnet on and off rhythmically so that the diaphragm vibrates. In fact, it uses the diaphragm's position to control the power to the electromagnet. Whenever the diaphragm is in its resting position or even farther from the electromagnet, a switch closes to deliver electric current to the electromagnet. The electromagnet then attracts the diaphragm's center. But when the diaphragm moves closer to the electromagnet, as the result of this attraction, the switch opens and current stops flowing to the electromagnet. Because of this arrangement, the diaphragm moves in and out and turns the electromagnet off and on as it does. The diaphragm's tone is determined by the natural resonances of its surface.

As soon as you ground that 2nd terminal the voltage would fluctuate up and down at the frequency of the vibrating horn. If there is something wrong in the horn to keep it in the resting position (ie rusted) it would be a dead short and cause the horn button spring to get too hot.

Paul