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Do you mean someone who delivered something to your house or to another house? That sounds unlikely to me, as surely that would be easy to trace, e.g. by asking your neighbours?
Definitely an opportunist, but I wouldn't discount the possibility that it's someone who knows the lie of the land very well. Also, a lot of thefts happen because someone observes that a bike is left in the same place all the time.
Thinking it was likely a delivery driver. The garden is too exposed for opportunists but not exposed enough to know that it had been left open unless you went into the garden, if you know what I mean. Or it was some brazen prick. Either way, I feel both stupid and fucking furious.