Hi all,
Should anyone face the same problem, I found the root cause: brownout threshold. I burned a new bootloader (Optiboot 8.0) without such trigger and the node has been working with used batteries (~2.7V) since May.
Hope this helps.
Sorry, no. Realised modern phones BT goes into sleep mode which means they don't announce themselves for longer periods. Kind of made it meaningless for the application.