If your pump runs every time you start the car yet you are not activating ABS everytime you drive, then there is probably a slow internal leak from the high pressure side of the system. Most often this is caused by a solenoid that is not seating quite right. It can also be a bad pump or a problem with the accumulator.
The pump of course only runs when it sees the need to repressurize the system. If you are not activating ABS frequently, the only reason it would need to repressurize that often is if it is slowly leaking pressure internally.
First try flushing the ABS brake fluid. This will often help with the stuck-solenoid problem. If that doesn't work, either take it to an NSX technician or get the manual and start troubleshooting.