I've had my Icom IC-7000 for over 4 years and ever since I bought it new, the S-meter has always gone down after the radio has warmed up. I had never been sure whether this was just the meter that was dropping or the receiver becoming less sensitive as it warmed up.
I wanted to test whether the receiver was really getting worse, so I found a signal on UHF which was on the air almost 24 hours a day and which mostly transmitted a dead carrier. This was reading about S9 but could be reduced to S1 by turning off the preamp and switching in the attenuator.
I recorded a few seconds of this signal and then a few seconds of noise from the adjacent channel (which was not in use) and compared the RMS levels of the noise, using Sound Forge. There was around 29dB difference between the unused channel and the S1 unmodulated carrier.
After leaving the Icom on receiving for a few hours, giving it time to warm up, I repeated this test. Although the signal meter had gone down to zero (with the attenuator on), the ratio of signal to noise was within 0.5dB of the first measurement.
So the receiver sensitivity doesn't seem to be changing but the meter is.