I have a very large project which even when nothing is playing, I am notcing there is action on the Master fader, down about -100db. Exporting a silent part of the song (eg 2 bars when nothing is playing) and the resulting wave looks silent, but on normalizing its a contant buzzing sound or whote noise static.
I have been working with a plugin developer to eliminate a couple of bugs that occur only with Cubase. He was perplexed as to why Steinberg was sending very small amounts of audio from channels when none was playing. After he got everything else sorted out, the meter was responding to this, so he suggested he might just have it ignore any signal below -90db. “I don’t know why Steinberg does this.” he mused.
That said, it doesn’t appear at all on Cubase meters, so when I see level appearing there on stop I always check whether a plug has some kind of analog emulation switches. As Grim mentioned Waves is often the cause (H-delay for example) or UAD Studer 800 which actually has a switch labelled noise. Others introduce line hum just to complete their classic emulations, which you can switch off.