diff --git a/.github/workflows/_checks.yml b/.github/workflows/_checks.yml
index 18eccf9..9c62c17 100644
--- a/.github/workflows/_checks.yml
+++ b/.github/workflows/_checks.yml
@@ -68,6 +68,8 @@ jobs:
       - run: uv python pin 3.12
       - run: just install
       - run: podman --version
+      # THROWAWAY (#105 measurement, not for merge).
+      - run: ./probe_podman_volumes.sh
       - run: just test-integration
 
   conformance:
diff --git a/probe_podman_volumes.sh b/probe_podman_volumes.sh
new file mode 100755
index 0000000..fbc494f
--- /dev/null
+++ b/probe_podman_volumes.sh
@@ -0,0 +1,47 @@
+#!/bin/sh
+# THROWAWAY -- measurement for issue #105, not for merge.
+#
+# Records what real podman does with the -v specs compose2pod would emit for
+# the drive-adjacent spellings `docker compose config` accepts. The ruling in
+# #105 (legitimate rule-two refusal vs. accept-and-defer) turns on these exit
+# codes, and no machine in the loop has podman except this CI job.
+set -u
+
+IMAGE=docker.io/library/alpine:3
+podman pull -q "$IMAGE" || exit 1
+
+mkdir -p /tmp/probe-src '/tmp/C:\data'
+
+probe() {
+  label=$1
+  spec=$2
+  output=$(podman run --rm -v "$spec" "$IMAGE" true 2>&1)
+  code=$?
+  printf '%-22s spec=[%s]\n' "$label" "$spec"
+  printf '%-22s exit=%s %s\n\n' "" "$code" "$(printf '%s' "$output" | tr '\n' ' ' | cut -c1-200)"
+}
+
+echo "=== podman version ==="
+podman --version
+echo
+echo "=== volume spec probes ==="
+
+# The form 0.4.4 now accepts (#104): does podman express it at all?
+probe accepted_backslash 'C:\data:/var'
+probe accepted_forward 'C:/data:/var'
+
+# The three residuals catalogued in #105.
+probe no_target 'C:\data'
+probe drive_relative 'C:data:/var'
+probe drive_relative_abs "$PWD/C:data:/var"
+probe single_letter 'a:/var'
+
+# Does a colon inside an absolute source survive podman's own split?
+probe abs_source_with_colon '/tmp/C:\data:/var'
+
+# Controls: the two shapes compose2pod emits every day.
+probe control_bind '/tmp/probe-src:/var'
+probe control_named 'probevol:/var'
+
+echo "=== volumes podman created along the way ==="
+podman volume ls