Are you finding these in home by the ocean where there is allot of salt in the air?
I have found a few but in every case it wasn't the light that caused the damage, it was the fact the circuit was spliced on the receptacle, some even stabbed in the back, and this was where the home owner had a heavy load down stream from this outlet, such as a window air unit or a portable heater, vacuum cleaner, almost all the cases was a bad connection at the receptacle, which started producing heat and eventually burned up the connections and receptacle, I could not see a 4 watt night light doing this, you might want to look on the next one for heavy loads down stream from the affected receptacles or ask if they used any in the near past. the heat generated from a bad connection with a 4 watt load would not be enough to cause much damage?
Remember the heat cause at one point can be transmitted to another part through the copper and aluminum connections, and can appear to be coming from the night light but in fact is a bad connection on the receptacle.