Inside a Logitech Trackball Marble Mouse:

There are always hidden screws or tabs in everything :(
I bet just looking at a screw hidden under a label voids any warranty.
This mouse has a hidden screw deep behind the label and is long past any warranty.

Under the cover, the cup that holds the ball has three tiny white knobs that the ball slides on, and a dark window in the back for the sensor.

Seems simple; cup, chip, sensor, 2 capacitors, 4 buttons, 2 LEDs, USB socket.

Ok, not that simple...

At first glance, I didn't notice the surface mount components on the bottom due to the deceptive retro through hole top.

The clear plastic near the bottom of the cup is a lens, magnifying the printed pattern on the ball. I didn't expect to see a lens until I risked breaking the sensor first. Funny it faces down; probably used for calibration before it leaves the factory. The sensor and LEDs were installed into plastic holders behind the cup window, then the circuit board was soldered into place, making it seem removal of this assembly might possibly mess up its alignment.

In my lack of planning, I was thinking I could just rig up my own cup and sensor holder with a rotory tool, knife, glue, and figure the rest out later.

The long term plan is to make an Open Source Trackball from scratch and get away from product hacking. For now, using the inner assembly as seen here seems to match the mouse project well at this stage.