/lib/udev/rules.d/50-bluetooth-hci-auto-poweron.rules is in bluez 5.37-0ubuntu5.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 | # Set bluetooth power up
ACTION=="add", SUBSYSTEM=="bluetooth", KERNEL=="hci[0-9]*", RUN+="/bin/hciconfig %k up"
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