Extremely LOUD squeeking on startup

Yesterday when my sister-in-law was leaving I was walking away when she started her car. After about a second of running she put the car in reverse and while she was backing up an extremely loud squeeking started. I told her to stop and turn off the car, felt the serpentine belt took a look at it and it was good. Tension and wear were okay.

She said it has been happening lately when she starts the car. I told her to restart it but we could not reproduce the noise and all of the pulleys that the serpentine belt was on were moving well.

Does anyone have any idea what it could be?

I think she has a 2001 Scion (Hybrid)... and I'm not sure of which model.

there is no scion Hybrid....