The initial three beeps (rising in pitch) are the ESC detecting each phase of the connected motor. Once the phases are detected, the ESC initialises to arm itself, producing a low sound (when it detects a valid (1100-1900us pulse-duration) signal), followed by a high sound (when it detects a valid “stopped” (1500us pulse-duration) signal). Once all five beeps have occurred, the ESC is armed and ready to be controlled. There’s a visual display of this and the other possible beep sequences in the firmware manual in Technical Details / Documents on the Basic ESC product page.
If you stop providing a valid signal, for long enough that the ESC notices, then once the signal re-appears it will redo the full startup process (detect the motor phases, then initialise and arm, including all five beeps). If the signal never stops then re-detection and re-initialisation don’t need to occur, it will just continue operating normally (since it never stopped being controlled).
This should beep at least three times (detecting the motor phases), and if you’re providing a valid stopped signal it should also beep the other two times, not just once. If I have an ESC plugged in to my vehicle autopilot board (sending a constant stopped signal), and I pull out the signal+ground pair, then a few seconds after I plug it back in the full startup sequence occurs.
Given you’re using an external driving board, do you know what happens to the PWM output when your program stops? Does it command it to turn off or something when it’s closing down, or does the PCA board just keep the signal going?