Tech Specs

High Level (V0)

Base Hardware After some preliminary research we shortlist the M5StickC to be a suitable candidate for mass adoption for the following reasons:

Opensourced design (easy to inspect and reverse engineer)
Easy usage: Quick Start
Mass availability (various sources, >1000 qty)
Small, low-cost (0) and extensible
Singaporean founder based MCU (ESP32-WROOM, Espressif) with BT/BLE/WiFi
Comes with various desirble features OutOfTheBox (OOTB): including accelerometer, clock battery, usb support, expansion, OLED, buttons
Potentially able to quickly target the M5Stack too for extended purposes
Target to last at least 16h without extra battery
Collected data stored in 1MB on-flash partiton
Primary factors for ESP32 include cost, availability, openly available libraries, specifications and most importantly an open and familiar toolchain.
Possible to target a larger device (M5 Stack FIRE / Grey)
Available GPS module
16M flash (instead of the usual 4M)
Larger battery

Power Consumption

A 16h projected runtime gives us an average current budget of about 5mA
MCU power consumption estimation w/ modem sleep:
Idle: 1mA
1s broadcast interval: 2.6mA
1s scan duration, 60s period: 1.6% duty cycle: average 2.4mA
1s scan duration, 1s broadcast interval, 60s period: avg ~3.66mA
Ideally we should use prime periods, but thats another story
Ref
Overhead is given to cover other losses (to be tested) from components such as regulators, led, accelerometer, etc. Other optimizations to be considered:

Casing

The existing form factor is only sufficient for mounting but not so much for usability and clipping/strapping onto things.
There is a need to design a 3d-printable external chassis with the following features:
Belt loop - to slide onto belts
Covers most holes/gaps but leave buttons reachable - improve weatherproofing
Could be designed in 2 halves and secured w/ up to 3 screws.
M5Stick:
Space for optional 3x Lp602040 450mAh cells (connected in parallel with onboard 80mAh cell) for extended runtime (optional)
M5Stack:
Space for 1700mAh (2x 850) additional battery (optional)

Software

ESP32-IDF cross referenced w/ stock code
BlueTrace protocol
Power optimization
Using MPU9250 for motion sensing and snooze (disable radio)
Using display for diagnostics on button press (storage usage, steps, encounters, etc)
High power mode (via attached charger/cable):
Serial interface for configuration and data retrieval
Button-press to enter screen
Diag: displays diagnostics
[stretch goal] Config: configures unique id and wifi parameters via audio coupling (static site/app to generate parameters)
[stretch goal] Upload data over wifi to an api endpoint

Extensibility

Such a design could potentially be adapted to other deployment models and use cases, when desired:
Placed on a public transport
Placed at a mall entrance
Solar powered
Attached to an external power bank
Attached or integrated into other objects e.g. school bags, shoes, helmets
Additional sensors/devices could be added to spare ports
GPS
Larger flash devices for longer storage (up to 16MB)
FLIR (for long-range temperature taking)
digital temperature sensor

Manufacturing Pipeline

Small volume (<= 1000 units)
Use M5stick SKU
3d print addons/modifications
Large volume (>1000 units)
Collaborate with M5Systems for scale
addons/modifications could be molded, rolled out as part of manufacturing pipeline
additional power/performance optimizations