Thought I had ruined my D700 sensor
Cleaned sensor; took shot of a white door to review, but camera wouldn't fire without something to focus on. Included part of the knob. Stuff still there.
The D700 has a menu setting (User choice) for the AF that either tells the camera to shoot whether or not it's in focus or to fire only when the AF has reached focus. It sounds like you have it set to fire only when the AF has achieved focus. Shooting at a smooth blank object - such as your door - probably didn't allow the camera to focus as it couldn't find any edge to work with. If so, you can depress the shutter release all you want and no shot will be taken. The presence of that piece of doorknob doesn't guarantee that the AF will pick it up, since the knob may have been just outside any of the focus points you have the camera set to.
As for your sensor cleaning strategy: Invest in one of those made-for-cameras sensor magnifying devices. They fit over the lens mounting opening and come equipped with built-in LEDs that illuminate the sensor and let you see every bit of dust present. There are several on the market for you to choose from.