• Lock it properly, keep an eye on it, and if you can't do the both, go home or travel to such occasions with a bike that is un-theft-worthy or you don't mind (too much) having stolen.

    On a Brompton there are two plainly viewable serial numbers engraved by the factory on both the main frame and the folding swing-arm.
    Pity your friend didn't take these down, (irrefutable proof of ownership) and pity they didn't chase down the (their) potentially stolen bike.

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