Brake sensor?

'97 Blazer. I'm noticing a squeal from the front end. I can only hear it at very slow speed when the window is down. When I depress the brake (but, I'm still moving) the squeal stops. As soon as I release the brake I hear it again. I'm told it's a brake sensor telling me it's time for brake service. Is that true? If so, it doesn't make sense that the sensor "deactivates" (and the squeal stops) when I depress the brake.
 
Thanks for any insight!
 
Karen

Not a sensor per say, but the scrapper on the pad.  It is a wear indicator that will make noise before the pad material is all the way gone, theoretically warning you before you damage the rotor.

So, to clarify, this scrapper or wear indicator is "engaged" (makes the squealing noise) when the car is in motion and is "disengaged" (stops making noise) when the brake is applied?
 
Thanks!

Due to pad flex, yes, this can happen as you describe it.  The longer you drive it, the more constant the noise will become.