The gain/atten settings getting changed is very possible and likely the cause. Based on your other responses, it’s safe to assume this unit is exposed to the elements? (at least temperature variation) The Spike software will auto reconfigure the device when it experiences a 2C shift in internal temperature. That shift might necessitate different temperature corrections as well as a different gain/atten to maintain optimal measurements. If either the gain or atten changed due to that automatic update, it would result in ~5dB shift in the noise floor. The signal power of an input signal should not change (other than correcting any small amount of error that might have been present with the old temperature correction).
At room temp, I would not expect this auto reconfigure to occur once the unit has stabilized in temperature.
I know you are on an older version of Spike, but newer versions do allow you to disable the auto recal in the preferences menu. If you do this, you subject yourself to measurement errors if the devices drifts too far in temperature. You can also force the gain/atten, which would still perform the recalibration, but might not update the gain/atten to eek out more dynamic range when it could, or may not provide enough room for a full scale input. Both of these would be minor repercussions.
