Strange voltages in my office space
You are probably measruing these voltages with a high impedance voltmeter.  That means even floating wires will have some voltage on them due to capacitive pickup.  Some of the light switches may actually switch some of these wires.  Others will just change the AC in the environment that is picked up by floating conductors.

To get a useful measurement, you have to put some load on whatever you are measuring voltage of.  Something like a 100 k&Omega; 1 W resistor accross the voltmeter when taking measurements should do it.  You may still get a few Volts from wires that are floating, but such readings should be low enough to tell you clearly there is no power available.