Red Flags
Even though the neighbors were just idly chatting about nothing in particular, they started to get the idea that the couple truly did have something to hide from them. As they contemplated what it was that they were working so hard to keep a secret, alarm bells started to ring in their heads.
They were concerned about the actions that their neighbors could be taking and the effects that those actions would have on their community. Given that smoke was being released into the atmosphere, it was reasonable to presume that it might be hazardous. The neighbors continued to talk, which led to their growing concern about the situation.