I think I’ve found the best place to hide a tracker on my Brompton: in its front carrier block!
It does require a Pebblebee Tag, as AirTag and Chipolo One are too big to fit. Just make sure to leave a space on top for the locking mechanism to fit.
Edit: I was told that AirTags fit as well, so I guess Motorola Moto Tag might fit too, as I believe they are roughly the size of AirTags.
@Blue_Morpho I don’t think it would interfere with the signal as the carrier block is mostly made of plastic. I haven’t tried the range yet, but I’m not too worried.