permanent 5 o clock shadow?

I used to have a pretty faint but still noticeable mustache, I have dark brown hair for context. I’ve been tweezing my upper lip hair by hand for a couple months now, and I’m always really careful to not accidentally snap a hair or pinch my skin. I exfoliate the area before and moisturize after and I haven’t noticed much any to my skin. However, even if I’ve scoured my entire upper lip for even the whisper of a hair, my friends tell me I have a little bit of a shadow. Apparently it’s not super noticeable but if you are looking it’s there.

I don’t understand how I can have a shadow without a hair in sight!! i’ve checked, it’s not my upper lip casting an actual shadow. And when i kind of push the skin out with my tongue there isn’t any shadow, so I don’t think it’s hyperpigmentation? Idk, any advice?