I've found this very upsetting issue by trying to use my Arduino Nano with a battery.
It happens also with the Battery Monitor sample code provided.
Try to upload it on your board, remove the USB cable from your computer and attach it on a power bank, or another device. By using a simple BTLe scanner you won't be able to see it.
If the Arduino is connected again to the computer with the IDE installed. Re open the IDE turn on the Monitor and the board will be discovered.
Another more simple test. Right after you have programmed your board, check with a BTLe scanner, you will see that the board is there. Quite the IDE, unplug the board, plug the board again, use a BT scanner the board won't be listed.
Is my board defective?
Does it happen to anyone else?
Am I missing something something?
I've made also a video: At the beginning I have the board plugged, I show that is not present in the list.
I open the IDE and the monitor: board shows up.
Then I close the IDE, unplug the IDE and plug it again, the board doesn't shows up.