Constant low charge in PCB even when disconnected
If you're using a digital multimeter on DC range the input impedance will typically be 10MΩ. This makes the meter sensitive enough to give ghost readings when connected to isolated circuits.

Try connecting a 10kΩ resistor between the probes and repeat the reading. You should find that any difference in potential is discharged fairly quickly through the resistor and the meter will read close to zero.

Interestingly, Fluke have a range of meters with a Lo-Z range to avoid this problem on DC and AC circuits. The input impedance is about 38kΩ (from memory). This is low enough to give a zero reading when measuring voltage on an open circuit even in the presence of 'noise' on adjacent lines.