diff --git a/core/dr-catalog/examples/sync_probe.rs b/core/dr-catalog/examples/sync_probe.rs
new file mode 100644
index 0000000..b79026c
--- /dev/null
+++ b/core/dr-catalog/examples/sync_probe.rs
@@ -0,0 +1,111 @@
+//! What a second device ends up with after adopting this library.
+//!
+//! Stands up an empty catalog, gives it the images the real one has, adopts the
+//! face shards into it exactly as a sync would, merges the real catalog in as a
+//! remote — and then counts. The point is to answer "why does the tablet show
+//! fewer faces for this person" without needing the tablet.
+//!
+//!   cargo run -p dr-catalog --example sync_probe -- CATALOG.sqlite FACES_DIR
+
+use std::path::PathBuf;
+
+use dr_catalog::face_shard::{self, FaceShardStore};
+use dr_catalog::Catalog;
+
+const MODEL: &str = "w600k_mbf";
+
+fn main() {
+    let args: Vec<String> = std::env::args().skip(1).collect();
+    if args.len() < 2 {
+        eprintln!("usage: sync_probe CATALOG.sqlite FACES_DIR");
+        std::process::exit(2);
+    }
+    let source = PathBuf::from(&args[0]);
+    let faces_dir = PathBuf::from(&args[1]);
+
+    let dir = std::env::temp_dir().join(format!("dr-sync-probe-{}", std::process::id()));
+    let _ = std::fs::remove_dir_all(&dir);
+    std::fs::create_dir_all(&dir).unwrap();
+    let dest = dir.join("catalog.sqlite");
+
+    let far = Catalog::open(&dest).expect("fresh catalog");
+    let conn = far.connection();
+
+    // The images a scan would have found. Nothing else: no faces, no people.
+    conn.execute(
+        "ATTACH DATABASE ?1 AS src",
+        [source.to_string_lossy().as_ref()],
+    )
+    .unwrap();
+    // Foreign keys off for the copy: `images` carries self-references
+    // (`shadowed_by`) that are only consistent once every row is in, and this
+    // is a bulk clone rather than an edit.
+    conn.execute_batch(
+        "PRAGMA foreign_keys = OFF;
+         INSERT INTO roots SELECT * FROM src.roots;
+         INSERT INTO images SELECT * FROM src.images;
+         INSERT INTO remote SELECT * FROM src.remote;
+         PRAGMA foreign_keys = ON;",
+    )
+    .unwrap();
+    let images: i64 = conn
+        .query_row("SELECT COUNT(*) FROM images", [], |r| r.get(0))
+        .unwrap();
+    conn.execute_batch("DETACH DATABASE src").unwrap();
+    println!("second device starts with {images} image(s), no faces");
+
+    // Adopt every shard, which is what a completed face sync leaves behind.
+    let store = FaceShardStore::open(&faces_dir).expect("shard store");
+    let adopted = face_shard::import_from_shards(conn, &store, MODEL).expect("import");
+    let faces: i64 = conn
+        .query_row("SELECT COUNT(*) FROM faces", [], |r| r.get(0))
+        .unwrap();
+    println!("adopted {adopted} image(s) from the shards -> {faces} face(s)");
+
+    // Then the catalog merge, which is where people and their judgements come.
+    let report = dr_catalog::sync::merge_remote(conn, &source).expect("merge");
+    println!(
+        "merge: {} people in, {} updated, {} kept local, {} face(s) assigned, \
+         {} kept local, {} rejection(s)",
+        report.people_inserted,
+        report.people_updated,
+        report.people_kept_local,
+        report.faces_assigned,
+        report.faces_kept_local,
+        report.faces_rejected,
+    );
+
+    // Per person, against what the source holds.
+    conn.execute(
+        "ATTACH DATABASE ?1 AS src",
+        [source.to_string_lossy().as_ref()],
+    )
+    .unwrap();
+    let mut q = conn
+        .prepare(
+            "SELECT p.name,
+                    (SELECT COUNT(*) FROM src.face_person sfp
+                      JOIN src.people sp ON sp.id = sfp.person_id
+                     WHERE sp.uuid = p.uuid) AS there,
+                    (SELECT COUNT(*) FROM face_person fp WHERE fp.person_id = p.id) AS here
+               FROM people p
+              WHERE p.name != ''
+              ORDER BY there DESC LIMIT 12",
+        )
+        .unwrap();
+    println!("\n{:<24} {:>8} {:>8}", "person", "source", "here");
+    let rows = q
+        .query_map([], |r| {
+            Ok((
+                r.get::<_, String>(0)?,
+                r.get::<_, i64>(1)?,
+                r.get::<_, i64>(2)?,
+            ))
+        })
+        .unwrap();
+    for row in rows.flatten() {
+        println!("{:<24} {:>8} {:>8}", row.0, row.1, row.2);
+    }
+
+    println!("\nprobe catalog left at {}", dest.display());
+}
diff --git a/docs/traceability.md b/docs/traceability.md
index e1c2e53..ca44ba4 100644
--- a/docs/traceability.md
+++ b/docs/traceability.md
@@ -9,7 +9,7 @@ Denominators are parsed from [`requirements.md`](requirements.md) at run time, n
 
 | Metric | Value |
 |---|---|
-| Source files scanned | 277 |
+| Source files scanned | 278 |
 | TRACES tags found | 811 |
 | Requirements defined | 177 |
 | Requirements covered | 106 |