The Banquet Alibi and the Cell-Site Records
Sunrise Maintenance Ltd is suspected of paying cash 'site allowances' to client-side inspectors. The disputed meeting is on 14 March, between 22:05 and 22:31, at Sunrise's site office in Fo Tan. Mr Tam Chi-wai, a government inspector, says he was at his cousin's wedding banquet in a Tsim Sha Tsui restaurant from 19:00 until about 23:00 and never left. Records show his handset registering with a base station serving the banquet area at 19:12, 20:40 and 21:38; with a base station serving Sha Tin at 22:05 and 22:31; and again with the banquet-area station at 23:04. The banquet bill was settled at 22:50 with Tam's credit card, by his younger brother. A waiter recalls seeing Tam 'around 22:40'. A taxi receipt in Tam's car is timed 21:52, Tsim Sha Tsui to Fo Tan, fare HK$213. Sunrise's office is a six-minute walk from that drop-off point. Night driving between Tsim Sha Tsui and Fo Tan takes about 22 to 28 minutes. HK$30,000 in cash was later found in Tam's locker, still banded with a bank slip from a branch 400 metres from Sunrise's office.
1. Test the alibi against the records. State what is consistent, what is inconsistent, and how strong the contradiction is.
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Reconstructed timeline
- 21:38 - handset in Tsim Sha Tsui. Consistent with him being at the banquet.
- 21:52 - taxi receipt, Tsim Sha Tsui to Fo Tan, HK$213. Consistent with him leaving the banquet just after 21:38. Fare level is plausible for the distance.
- 22:05 and 22:31 - handset registering with a Sha Tin base station. Roughly consistent: a car heading to Fo Tan via the Lion Rock Tunnel passes through Sha Tin, and he would arrive at the drop-off about 22:14-22:20, reaching Sunrise's office around 22:20-22:26 - squarely inside the disputed 22:05-22:31 window.
- 23:04 - handset back on the banquet-area station. Leaving Fo Tan at about 22:31, he would reach Tsim Sha Tsui at about 22:53-22:59. Consistent.
The contradiction. Tam's account is that he never left between 19:00 and 23:00. The taxi receipt and the Sha Tin registrations are inconsistent with that. But the strength of the contradiction does not rest on the waiter: a single recollection, given later, of 'around 22:40' in a crowded banquet, is weak identification evidence and cannot bear much weight. The 22:50 bill is not evidence of Tam's presence at all - his brother settled it using Tam's card, and the card may have been left with the brother.
Assessment. The records are consistent with one coherent sequence: Tam left the banquet at about 21:40, travelled to Fo Tan, was there roughly 22:20-22:35, and returned by about 23:00. That is a serious challenge to the alibi, but it depends on the authenticity of the taxi receipt and on the precision of base-station evidence, both of which must be nailed down before the alibi is put as false.
2. Which further evidence would you seek, and which single item would most strengthen the inference?
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Further evidence
- Taxi company dispatch and GPS record for the receipt number, and the drop-off timestamp - establishes whether the journey happened and when it ended.
- CCTV at the restaurant entrance, lobby and car park, time-synchronised, covering 21:30 to 23:10 - shows whether Tam left and returned, and whether anyone was with him.
- Coverage and sector data for the Sha Tin base station, plus an expert statement - establishes whether the 22:05 and 22:31 registrations are consistent with the Fo Tan route and excludes nearby alternative locations.
- Sunrise's office access log, door CCTV and staff present that night - places or excludes Tam at the meeting.
- The bank band: serial number, issuing branch and withdrawal date; branch CCTV and the withdrawal slip - shows when and by whom the HK$30,000 was drawn.
- The brother's account of the evening, and whether he or anyone else had Tam's phone and card.
The single most useful item: time-stamped CCTV of Tam leaving the restaurant and returning. It converts a probabilistic inference from cell-site data into a direct visual sequence, and it independently fixes whether he was present at the moment the bill was settled. If such footage does not exist, the taxi company's GPS record is the next best, because it verifies or destroys the receipt on which the whole reconstruction depends.
3. A junior officer argues: 'The waiter saw him at 22:40, so he could not have been in Fo Tan, so the taxi receipt must be forged.' A second argues: 'The phone was in Sha Tin at 22:05, so Tam was in Sha Tin at 22:05.' Identify the reasoning errors in each.
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First argument - false dichotomy plus a weak premise. It treats the waiter's 'around 22:40' as a fixed, reliable time and as mutually exclusive with the Fo Tan trip. But (i) a recalled time is approximate and may easily be 20 minutes out; (ii) a person leaving the banquet at 21:40 and returning at about 23:00 was, in fact, at the banquet both before and after the trip, so 'seen at the banquet' and 'went to Fo Tan' are not contradictory propositions; and (iii) even if the waiter were right and the trip impossible, that would make the receipt unexplained, not forged - forgery is a separate conclusion needing its own evidence. The argument also reasons from a conclusion it wants (Tam is innocent) backwards to the evidence.
Second argument - treating a necessary condition as sufficient, and ignoring the technology. A handset registering with a base station is consistent with the handset being in the coverage area; it does not prove the handset was in a precise spot, and it does not prove who was holding it. Cells overlap, sectors point in different directions, and a registration can occur from a moving vehicle. The correct inference is only: 'the handset was somewhere within the coverage of that station at 22:05'. To say 'Tam was in Sha Tin' you need an additional premise - that Tam had the phone with him - which is exactly what is in dispute here and must be established independently (his own admissions, CCTV, a call he answered, a message he sent).
Both officers should be asked to write the inference as a syllogism; the missing premises become the investigation plan.