Eliminating millivolt-level reverse current flow with diode or other
Figure 1. The RL207 datasheet explains it.

If your multimeter has a 10 M&Omega; input impedance then 1 &mu;A will give V = IR = 1&mu; &times; 10M = 10 V. This is very close to what you are seeing.







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 2. (a) The circuit redrawn in a more conventional manner with + on top. (b) Loading the circuit.

Try adding a resistance to shunt the stray current away from the multimeter and measure the voltage again. Report back!