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.
@xtaran @sanji @brompton Yes, thanks for adding that bit of info I left out, I probably should edit with a picture of the assembled block.
I did not try, but the charging cable being as big as the tag should allow to charge it in place. It should last a year according to the constructor.