Hit Location Warning is a rule found at the start of page 109 of GURPS Zombies.
It says the -4 penalty to bite the hand of a person who tried (and failed) to grapple the neck or head does not apply on the following turn.
It doesn't have the most exact wording. The intent is probably only to ignore the penalty for a hand that was doing the grappling, but since it doesn't specify this, RAW it might be interpreted to apply to:
- bite the unarmored right hand of someone who tried to grapple a zombie using an armored left hand
- bite the hand of someone who tried to grapple using a weapon
- bite the hand of someone who tried to grapple using their legs
Zombies were not designed with any sort of trait to reflect this rule. It could not be designed as a Targeted Attack because those can never completely buy off a striking penalty.
Given that hit penalties CAN be completely bought off with grappling targeted attacks, designing it that way might make more sense, but that would mean the initial bite couldn't do any damage.