It was certainly a trace, I used BLE_APP_DISABLE_TRACING() macro and now string is disappeared.
Anyway also application level traces are disabled.
Is there a way to keep application level traces enabled?
In your APPLICATION_INIT() function, if you set ble_trace_enable = 1, ble_trace* functions will print to the peripheral UART. If you set this to 2 instead, debug traces will go to the HCI UART. Since the 2073X ROM uses HCI UART RXd to detect the presence of a host device (for download mode), make sure that you disconnect RXd line, leaving the TXd line connected and then power cycle the device (or the device will enter download mode again).
Setting ble_trace_enable = 0 instead will disable all debug messages.
I do see that we now include a simpler function within APPLICATION_INIT() to disable traces per what I just pulled out of hello_sensor.c in SDK 2.2
// BLE_APP_DISABLE_TRACING(); ////// Uncomment to disable all tracing