I'm sure there are metaphors to be made from what I'm about to talk about but for now I'm just going to give you the literal story and let you fill in the metaphors as you see fit.
It used to be, for a long while, that there was a person who lived at the end of the dead end road I can see (and hear) from my apartment who had a loud (Harley Davidson) motorbike.
I knew he had this motorbike because he would REV THE THING incredibly loudly as often as he wanted. Leaving. Coming back. At whatever hours. He would make the most obnoxious, impossible to miss noise and it quite often rubbed me the wrong way. (I think being unnecessarily offensively loud is rude... to put it politely.)
Sometimes I'd walk past the motorbike and have the thought to push it over as a symbol of my annoyance from being woken up or disturbed by the ROARing of the pipes. But I never did. (But I thought about it.. in anger and frustration.)
I once saw the guy on his bike with a giant grin on his face (an older fellow for whatever that's worth) and I tried to soften my frustration thinking how happy he must be on his bike but after chatting with a friend or two who have motorcycles and learning that there is no necessary reason to be making that much noise on purpose (I thought maybe it helped clear like soot out of pipes or something, I don't know, I was trying to be kind) I was just annoyed.
A couple of weeks ago my neighbour (the one I am friendly with) randomly messaged me about how the motorbike guy had moved. She walks down that way to work every day and so had noticed the bike not being parked (on the lawn) and the piece of wood from the lawn (that kept the kickstand from sinking in to the grass) was also gone.
And I was surprised. (I'm still cautiously not being thrilled in case he's just away for a while...it being the warm/dry season after all...)
I was surprised because I hadn't noticed. (And this is where that metaphor comes in by the way.) I hadn't noticed the lack of noise. I hadn't noticed not being annoyed a few times a day by the loud motorbike. It stopped happening and I didn't register the lack of it.
My nervous system maybe did, somehow, deep in there, but I, on the surface, didn't.
I'm sure it's because it wasn't regular like clockwork or anything but still... it was interesting to me to notice that I hadn't notice the cessation of something that I didn't like.
(Now go apply that to some other things in your world/life, eh?)