Nancy has a bit of problem facing her. She can look at this problem in one of two ways, the first being that all the money was in there and accounted for or she can think that there is some thing not right since the bills were all in consecutive serial order along with being new.
One of her first choices is that all the money was in the box and accounted for which technically means that there is nothing wrong and would probably be very hard to prove. She would also need to know when the last petty cash checks were given out. The custodian could have just cashed the new check recently and that would explain why the bills were so new and the serials numbers were in order.
Second choice is to inform a her supervisor so the funds could be monitored more closely with more random checks.
This would be a very serious step to take because that would bring into question of how trust worthy the person is. The other thing she would have to take into consideration is did the person have enough time to go to the bank and get money from an account to cover the missing money.
I would say that if the person did not have enough time to leave or not seen leaving then I would say that everything was fine. If I could not verify those two things then the first step would be to check the petty cash on a more frequent time schedule but keep it very random.
One thing to take into consideration is that if the person in charge of the petty cash has to go cash the checks then it would not be uncommon that the bills be new and in order. If the person in charge of the petty cash is given a draw from the cash office then there would be a reason to watch more closely because that person is dipping into the petty cash.