Well, the host *should* notice at the end of the night that the box isn’t right. But that doesn’t mean s/he knows who cheated.
Say I was using generic injection mold chips, and someone snuck four green $25s into my 2/5 cash game, making the box $100 light at the end of the night.
As a host, the first time it happened I would probably assume that *I* screwed up, and miscounted a buy-in or rebuy, or had two $100s stick together at payout... I could have counted their cash wrong, or had a brain fart and given out too many chips.
So I would review my handling of the box, and be more careful the next time.
The second time it happened, I would be more suspicious. If I concluded it was someone introducing extra chips, I would not know who did it just because the box was light.
I could try to guess who it was, based on their personalities, or who cashed out for more than I expected. But the chips could have been snuck in by a losing player during the game and he busted, so tge cashouts alone wouldn’t tell me anything.
So then I’d have to start watching people’s movements and stack sizes like a hawk. Eventually the person would get caught.
But just having a light box would prove nothing.
One thing I might do is tell the whole group, “Hey folks, the box has been light two games in a row. I’ve been super careful to make sure all cash and chips match up. Everyone please triple-count the cash you hand me and the chips I give back, as I will be doing. I’d hate to think anyone was sneaking chips into their stacks.”
That would at least put the cheater on notice and with hope put an end to it, for a while at least. But I’d have to stay vigilant.