If I've understood correctly (and despite having used it many times, its operation is still confusing to me), the issue is that departing passengers can inadvertently become arriving passengers by staying on the transit at T5C, and this is the only way that this can happen. (A departing passenger who wants to become an arriving passenger within T5A needs to be escorted out; a departing passenger at T5B getting on a train can only get to T5C; and they remain a departing passenger whichever way up or down the walkway they go.)

Is that right?

If so, then perhaps it would make sense for HAL to carry out a quick check of the transit at T5C as they do at T5A, to ensure that everyone gets out before it starts its journey in the other direction.