Tripwire on the Threshold

I checked the survivor's crate behind the steel counter to confirm she remained undisturbed under the burlap. Night had settled over the toll plaza. Stepping down to the sliding glass doorway, I drew a discarded coil of monofilament nylon line from my pouch. Kneeling low, I knotted one end around a rusted anchor bolt by the sill and stretched it taut two inches above the asphalt threshold, tying the other end to an empty tin breath-mint can perched on an iron bracket. I tapped the line once with my finger. The tin rattled sharply. The entrance is rigged for early warning. Our position holds.