Join Date: Oct 2004
Location: Anywhere but home
Programs: UA 1K/MM, DL GM/MM, HH Dia, PC Plat, MR Gold, ALL Sil,
Posts: 4,553
It sounds like a lazy FA who didn't check match the number of FC passengers to what's on the manifest. You're right, it probably is nearly always full and so they may just not bother to check.
I had a friend who did this (without my blessing) on NW probably 10 years ago. I was Platinum at the time and had already been able to upgrade my brother. The FA caught my friend, but let him sit there anyway as a nicety to me as a second companion upgrade.