diff --git a/docs/FAQ.md b/docs/FAQ.md
index 5dafaac192..23f53d54ce 100644
--- a/docs/FAQ.md
+++ b/docs/FAQ.md
@@ -89,6 +89,8 @@ following conditions:
 will use a bottled version of the formula, but
 `brew install <formula> --enable-bar` will trigger a source build.
 * The `--build-from-source` option is invoked.
+* The environment variable `HOMEBREW_BUILD_FROM_SOURCE` is set
+(intended for developers only).
 * The machine is not running a supported version of macOS as all
 bottled builds are generated only for supported macOS versions.
 * Homebrew is installed to a prefix other than the standard