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Maybe Théo already met this person once already before, say to buy weed, and then on second meetup after he left CM, they spent more time together. If not Théo plus one male familiar with the area, then I can only think Theo, plus male and female backpacker went to Cosy Corner and some dispute, fight or overdose happened.
Police may have similar suspicions but lack evidence. But I keep wondering if we have Theo’s movements tracked so well, can’t they do same to link other person/s. If unintentional but not reported, mobile tracing should work.
If premeditated, planned, then perpetrator would not be using a mobile, switched it off, or doesn’t own one if homeless. This person probably not in CM, but nearby.
Third option, two dealers working together, one in CM setting up deals, the other outside closing the deal, maybe one or both went for a walk with Theo