I made a new project with the “HelloKeypad code”. I added in the Keypad.cpp and Keypad.h that I was using previously (that you talked about earlier in this thread).
The only changes I made to the code were the pin assignments (from Arduino to Spark) and replace the #include Keypad.h with a quoted one.
The behaviour remains similar. Output is being produced even when no key is pressed and intermittently when the keypad is disconnected.