Well, only 13 countries recognize Taiwan as a sovereign country - such as the 10,000 people of the 21 square kilometer island Naura.

Also, Taiwan occupies Kinmen Island in the bay of Xiamen's harbor on the PRC mainland. The island is in the Xiamen harbor and is 10 km from the city of Xiamen. So any PRC planes flying around Xiamen are "violating Taiwan's sovereign airspace" (which almost no one recognizes as "sovereign" - one of the main parties on Taiwan acknowledges Taiwan and the mainland are all one country).