The actual details for this question can be found on pages 136-138 of "My Brother Sam Is Dead." Tim's mother tells him to hide the pewter in the barn and when he finishes that chore he stops outside to observe. Tim says he first sees the vanguard and a drummer boy. He then sees the flag-bearer and some officers on horseback. Following them are the marching soldiers. He says, "it was a frightening thing to see."
After arriving in town the British officers go in to Mr. Heron's house and Tim assumes that he knew they were going to arrive. He imagines all of them sitting around eating and doesn't feel right about Mr. Heron feeding and meeting with the British.
Tim also believes the British officers are visiting with Mr. Heron so that he can tell them who the Tories are and who the Colonialist sympathizers are. He believes that Heron is selling information about the rebels and anyone who is not loyal to the King of England.