nt people do stim occasionally but most wouldnt pick up on it
and not necesarly for the same reasons
its more common things like toe tapping or leg bouncing /jiggling when stressed or nervouse or something
stimming is realley sesnsory equilization
its a really quiet room they may drum fingertips to add sound or in a really busy place they may hum to bottom out the noise with a constant baseline
its done for 3 reasons from what i can gather
to little input
too much input
or comfort
rubbing a textured toy when in a fairly quiet room or singing would be because of too little sensory input
the same in a very busy room is to fucus on one input at the cost of all others
a person who hums when upset would probobly be doing it for comfort
this is what ive gathered i may be wrong in some(probboly all ) aspects