Hi Mate:
A great case to apply Rule 62: All-In with Chips Found Behind Later:
If A bets all-in and a hidden chip is found behind after a player calls, the TD will determine if the chip behind is part of accepted action (Rule 52). If not part of the action, A is not paid off for the chip(s) if he or she wins. If A loses, he or she is not saved by the chip(s) and the TD may award the chip(s) to the winning caller.
So, using the above rule, do you think the chip(s) B found was reasonably part of the all-in action A accepted? If so, A is obligated to match that amount also.
OTOH, if you think the chip was "hidden behind and found later", then 1) A isn't obligated to pay B off for the chip(s), and 2) the chip(s) won't save B if he loses. This rule was adopted at the 2013 TDA Summit at the Venetian. Interestingly, the ticklish part of the rule is the last part where "the TD may award the chip to the winning caller", effectively meaning in your case that A could win the chip but wouldn't have to pay B off if A loses. This is left to TDs discretion and sensibilities.
Bottom line, as I read your post the picture I get is that the chip was truly hidden and A can't reasonably be said to have accepted it considering: 1) it was hidden and 2) it wasn't brought to anyone's attention until after A had called and tabled. But you were there, how would you have applied Rule 62 to this case?
Thanks for the very interesting example!