(a) Last Sunday, Sam was shopping in the market. He found a wallet in a trolley. He saw $100 in the wallet.
(b) Sam quickly went to the guard's house. [To me, this means that Sam went to the house where the guard lived. Is that what you want to say? The guard is more likely to be patrolling the market, I'd have thought. You may be thinking of "guardhouse", but a guardhouse