Unexpected Sensor Output Voltage Rise
If your system uses a photodiode for detection, it is attached to a relatively high gain amplifier/integrator, and strong electromagnetic fields (wifi) can result in induced AC voltages that are rectified by the diode junction and appear on the output.  If this is your problem, you can solve it by increasing the distance from your wifi transmitter or additional shielding around the photodiode.  I'm betting your sensor has some shielding around the photodiode already.