Absolutely no hair should be out of the safety gear. The last thing I do before firing the kart is a hair check. Typically if long hair is in a ponytail and tucked into the suit all is good. However, if even a strand is out (in our case blond) I take my greasy fingers and tuck it back in.
I believe IKF, and likely other organizations, require that no hair be outside the gear.
Should be an immediate black flag if hair ever comes out during a race or practice.
I would think the danger might be as great in the pits around running engines/turning axles so this would be another area to use caution and common sense…