Arduino Nano IoT Power Consumption
Two of those 10mA come from the ATECC608 crypto chip which idles at 2mA.  That leaves 8mA for you to track down. 

The WiFi-NINA module has multiple "sleep" modes that range from 30mA down to 4.5µA. Since you can't be in the 30mA "modem sleep" mode (it's too high for your measurements) I suspect it enables "light sleep" at 800µA. 

The switching regulator uses 200µA.  So those two together probably account for 1mA in total, leaving you 7mA more to find.

The accelerometer can be up to 1.25mA. I'll assume that since you don't mention it, you haven't set that to "power down" mode.  5.75mA left.

The main MCU is actually quite hungry for power, even when sleeping. Looking at the datasheet it can be between 1.3mA and 2.4mA for the different "idle" modes. Only "standby" mode (where clocks are switched off, etc) drops it below 1mA. With it running a simple program you are looking at 4-6mA.

So it could be that the MCU isn't actually sleeping properly, or that other modules that you think are in low power aren't.

I would suggest manually disabling chips completely (pull their respective RESET pins if available LOW if available) to see how that affects the current.