Why my Honda horn keep beeping when connecting negative battery terminal

Is the key and switch a matched pair? There is a chip in the key (black plastic grip). It must communicate with the switch for the car to start ...
https://www.rockauto.com/en/catalog/honda,2007,accord,2.4l+l4,1433638,ignition,ignition+lock+cylinder,10041

Where are you connecting the tester to? If you are connecting one lead to the negative terminal on the battery and the other to the horn wire and it lights up try connecting a wire from the horn housing (bare metal) to the negative battery terminal and attach the horn wire to the horn and try it. if that works the ground path from the battery to the horn mounting place is broken somewhere. you may have to run a negative wire to the horn mounting place to make it work properly. Hope this helps.
Bob

probably is the alarm being set off by a corrupted CPU program from connecting the leads incorrectly. When using jumper leads or jumper battery the positive lead is connected to the battery terminal first and the earth wire ( negative) is connected to the to the engine . NOT the battery. IT is also advisable to turn on the head lights These precautions prevent a power surge in the CPU that corrupts the program