The majority of bike thieves in Portland are not going to buy a $15-$20 RF detector because that would have to come out of their meth budget. They are not a tech-savvy lot.
I suspect some posters are imagining bike thieves as part of some kind of international black market for high end bikes...maybe that exists but 99% of bike thefts are done by the local crackhead who either wants a ride or wants to trade it for $20 for some crack or meth.