ST. LOUIS COUNTY • Orchard Lakes is a leafy bit of tranquil suburbia swept up in a mystery. Dangling lucrative offers, two developers hope to induce the subdivision’s residents to sell their 1960s ranch-style homes and leave the neighborhood of big trees and curving streets at the edge of Creve Coeur. Replaced by what, the developers aren’t saying. That prompts Orchard Lakes residents to speculate what might replace their well-kept homes, lawn sprinklers and quiet streets. Walmart? Target? Menards? Some other big-box store?