Account for unknown error in time series data
I would set up a sensor as idle, purposefully to determine the decay in the signal. Then try an ARIMA or similar model on the data collected to find out what an approximation of the decay function is. 

Further to this you should determine if the model still fits if they decay is put into a "stop/start" mode.

Afterwards you can apply a corrective function to remove or take into account the decay on your other data.