I couldn’t tell you if my dad has a moustache right now.
I see him every day, and I am not sure. Some people look very similar with one and without one.
I’m with GRAVITY on this one. Maybe no one said anything because you look goofy w/o a hairy lip. Not that you wouldn’t look normal a week from now, but when it’s a freshly shaven lip it may seem strange to folks.
Actually, there was an episode where Chandler get’s eyeglasses and nobody - not even Monica - notices.
I’ve got to say that this is really something that could easily go by unnoticed, as impossible as it sounds. I think we tend to view people in a specific way, and as long as those things don’t change, we really don’t pay attention.
My best friend in the world… hell, I don’t even know what his hair looks like. I seem to remember him being bald before… but now that I think about it, I don’t think he is now…
Other things you mention aren’t necessarily a drifitng apart for the worse, either, but a drifting apart because of other stuff. If you love her, try to get something going on. If that fails, well… 
You mean you think he might have a hairpiece, but you’re not sure? Damn…
I think it’s damn weird.
When I was a toddler, I was visiting my Uncle Colin for the first time in about a week. He was not someone I saw very regularly. It was the first time he wore his toupee when I was around.
Astonished, I said: “Uncle Col!.. HA-AIR!!!”
I’m female and, yes, I notice when someone has shaved off a mustache. I do have a tougher time if someone has a mustache AND a beard, but shaves off just the beard. But usually, I do notice. Particularly if it’s someone I see a lot.
When I was dating a guy, I always noticed changes in his facial hair. Goatee/no goatee/part of goatee/stupid-little-tuft-under-bottom-lip/five o’clock shadow. If he always had a mustache or goatee, I’d definitely have notice if it had gone missing.
I think it’s weird that a live-in GF didn’t notice.
This is more telling than not noticing the shave, as you’ve realized yourself.
The question is, now, you have to answer is do you want to revive your relationship? All long-term relationships ebb and flow. You’re obviously in the low-tide. Do you want to return to the more passionate relationship or just be friends now?
My uncle had a full beard and we used to work together every day and it took me three days to figure out what was different (when he shaved it all off). Similar thing happened when my dad shaved his mustache off. I knew something was different, but I just couldn’t put my finger on it. . . . .
Don’t worry about it, seriously.
Another question: did you guys kiss at any time in those three days? This, to me, is why it’s weird for her not to notice. I would notice something different about my wife’s lip much sooner than my dad’s because of the smooching. Not kissing once over a three day period is, to me, a warning sign bigger than not having sex for several months.
It sounds like it’s time for a talk with the SO about your relationship. Maybe you’re just in a slow spot right now, or maybe you’ve drifted apart and it’s time to move on. Good luck.