This is one of my long-time interests. Ever since people started walking around with bluetooth devices, I assumed it would be a target for analysis. (Not many years later, it and wifi became a popular target for analyzing behavior of customers in retail stores)
Another fun one is TPMS sensors (though now that APLR is common, it's probably pointless).
But simple HUMINT/FININT works amazingly if you're positioned to collect it
