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The Detective’s Desk

MEDIUM · CASE FILE

Between two silent sensors

Medium · COPPER SCREENING GALLERY

Four visitors claim to have crossed a corridor while both sensors were inactive. The sensor records use an exact boundary convention. Also reconstruct the review register: it records who examined which source, independently of whether their later proposed answer is correct. Review slots describe order, not travel or clock times.

Investigate this case

Read the case file below, or open the interactive desk to examine records, keep notes and submit your reasoning. These are the case materials, without the solution.

Your assignment

Whose crossing time fits both inactive periods?

People and accounts

Venn Bell

Proposed deduction

“I crossed at 15:28.”

Sela Gray

Proposed deduction

“I crossed at 15:18.”

Tavi Hart

Proposed deduction

“I crossed at 15:24.”

Umi Cole

Proposed deduction

“I crossed at 15:12.”

Case records

Read each record on its own terms. A claim made by a person is different from a fact established by an independent record.

CONTEXT NOTE

Corridor plan

The corridor is straight and contains no side doors.

No movement duration or route inference is needed for the stated crossing instants.

PRIMARY RECORD

East sensor log

The east sensor was inactive from 15:10 up to, but not including, 15:24. It was active at 15:24 and at all times outside that interval.

This gives one interval and excludes its ending boundary.

LINKED RECORD

West sensor log

The west sensor was inactive from 15:18 through 15:30, including both endpoints. All crossing times are accurate.

This provides the second interval with its own boundary rule.

RECONSTRUCTION RECORD

Review register

This is the same investigation’s document review record. There are 3 ordered events, labelled Review 1, Review 2, Review 3. Each listed person, location and item occurs exactly once. These slots describe record order, not times from the analytical problem.

  • The event reviewed by Umi Cole occupies slot 3 (Review 3).
  • The event at Reading desk occurs earlier than the event involving “Corridor plan”, without claiming they are consecutive.
  • The event involving “East sensor log” occurs immediately before the event at Review bench (consecutive rows, with no event between them).

Use each column’s choices exactly once. Verified relationships govern the board; proposed answers and later recollections do not override them.

RECONSTRUCTION RECORD

Review cross-check

The relationships below were verified independently. “A immediately before B” means A is in the row directly above B, with no row between them. Keep relationships from all worksheet sources together. The following later recollections are claims to test, NOT verified facts. Exactly one disagrees with the reconstructed board; this alone is not proof of deliberate deception.

  • The event at Reading desk occupies slot 1 (Review 1).
  • The event reviewed by Venn Bell occurs earlier than the event involving “East sensor log”, without claiming they are consecutive.

Use each column’s choices exactly once. Verified relationships govern the board; proposed answers and later recollections do not override them.

Questions to resolve

Match every recorded item to its handler or reviewer.

Whose supplementary location recollection conflicts with the board?

Use the interactive case desk to select your finding and cite the records that support it. The answer is checked there, and your progress stays in your browser.

Build your finding

Compare the records with each account. Work out what the evidence establishes, then separate that finding from anything it does not prove.

Open the interactive case More Medium case files