The OP specified that he wants to learn, but you make a good point when it comes to a sqeak, which can be hard to diagnose. But a large number of the current class of bike shop workers are a product of an educational system where logic and reasoning do not seem to be well emphasized, so they may be poorer at finding the cause than the OP is with some guidance.
To the OP. If you do take it in the shop needs the same info you do in order to quickly find and fix the problem.
When did the problem start? Did you make any equipment changes at that time?
Can you reproduce it at will or is it random?
Under what conditions (gear, pedal pressure, braking, etc) does it occur, and what is the frequency (once per pedal revolution, once per wheel revolution, every 2 pedal strokes, etc.)