No one was inside a Concord building when it caught fire early Wednesday morning. Concord firefighters say a passerby called around 1 a.m. to report smoke coming from a building on McGill Avenue. Firefighters arrived minutes later and were able to control the fire within 15 minutes, preventing the fire from spreading to other businesses. Fire officials say no one was inside the building at the time. Three firefighters were treated for dehydration. No other injuries were reported. The cause of the fire remains under investigation.