As a kid, I had a ticket on the top of my head just a bit back of center. Couldn't see it in a mirror, didn't feel anything weird right away. My mom noticed it at some point, but we had no idea how long it had been on there at that point. I was between 6-8 IIRC.

I always figured it's a sensory thing, different levels of skin sensitivity. I'm highly sensitive, so I imagine there are people who are much less sensitive. For me, I get tickled very easily (and I absolutely hate it.) Any light touch can trigger it. A gentle caress by someone's fingertips makes me want to jump out of my skin. Lately my skin's even become more sensitive to chemical irritants. I can't use the bandaids my boyfriend bought (which are pretty average and of a standard brand) because the glue in it now gives me a rash.

But the benefit of this sensitivity (and, I suspect, the reason this hyper-sensitivity was maintained in humans at all) is that I can feel ticks and most other bugs almost immediately. I'm not perfectly immune - they are sneaky little fuckers. Sometimes a lucky one is able to sneak onto my scalp undetected for a bit. I became aware of that after the last time it happened, so now I brush my hair and feel around my scalp as part of my "tick check" routine.