Went to high school with a Bunch of Asian dudes ago looked like Bruce and never trained a day in their life... Aside from working at families hole in the wall restaurants till they went to college. (Grew up in Viet/Lao/hmong neighborhood)
Since you weren't with them 24/7 you wouldn't know 100%.
I'm sure some of them training and lifted something more heavy than a plate the 20 minutes of the day you weren't inspecting them.
Someone could have a heavy book under his bed for example, someone could have done lats pullups at the playground for example
Or maybe a pushup or two before bed/after waking up.
You never know with those sneaky gits.