Pretty sure my cat weighs about 15lbs. Flown him Southwest across the country several times. The only time they even look towards the carrier is when I tell them I need to pay for a “carry on pet.” At that point they usually just say “awwww, he’s so cute” or something along those lines.
I always bring him on the metro and bus (to get to and from from the airport) and not a single person has ever noticed. Most just assume it is an extra bag/purse…unitl he meows.