Examine the time variable in the offline data. Any change over time in the characteristics of the signal caused by, e.g., reduced battery power in the measuring device as time goes by, or measurements taken on different days may be made by different people with different levels of accuracy. Also, examination of time can give insight into how the experiment was carried out. Were the positions close to each other measured at similar times? Do you see any change in the signal strength variability or mean over time? Try controlling for other variables that might affect this relationship.