How come UFO's have never created a sonic boom? That we know of anyway

Because they manipulate space time for propulsion rather than kenetic force. This creates no noise. Its not really moving its just super positioned in different points through reality until it gets somewhere

If they were real "space craft" they'd either fly above the atmosphere, or fly through it slowly so they can make closer observations?

(I don't believe they are the comic-book "alien craft"...)

Scalar energy may be the reason

Technology

My guess is they can bend space and time.

Because UFO's are not aircraft, which means a UFO does not rely on aerodynamic forces acting upon itself to enable itself to fly, which would otherwise create sonic booms if a UFO did rely on aerodynamic forces to remain in a state of flight while moving through the air at a speed which exceeded the speed of sound.

An aircraft is a machine or device that derives it's 'lift' from the surrounding air which supports it's weight while in a state of flight, which means that an aircraft can only fly within an atmosphere where air exists, which is also why it is called an "air craft".