When a pin is configured as a High Z, the pin is floating and yes, it is disconnected from the drive source. But there is a small leakage current present on the pin. This leakage current when combined with the very high impedance of the Oscillocope or Digital Multimeter probe will show some voltage. This is a perfectly normal behaviour. Connect a low value resistor in the order of a few 10's of kilo ohms between the pin and GND and this voltage will disappear.