for $5 range I compared with https://minew.en.alibaba.com/product/60606212990-802524400/Factory_Direct_Sale_Anti_losting_Tracker.html with nRF51822, buzzer, on/off button that is $3 at 1000 units (that was 2 years ago)
I like the http://www.m5stack.com/ form factor - small, 320x240 LCD, stackable, easy to prototype.
another cheap round design ESP32 solution – https://www.tindie.com/products/Sandeep_ee/hornbill-esp32-minima/
just brainstorming around
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