How to debug STM32L433 Current leakage
After spending much too much time on this here are the results.

When USB is put to a low power mode after suspend with 

void HAL_PCD_IRQHandler(PCD_HandleTypeDef *hpcd){
    .....
    hpcd-&gt;Instance-&gt;CNTR |= USB_CNTR_FSUSP;
    hpcd-&gt;Instance-&gt;CNTR |= USB_CNTR_LPMODE;
    ...


There is a remaining residual ~200uA caused by a floating USB DM pin.(Floating digital inputs draw power (Implications of Slow or Floating CMOS Inputs)

The solution is to use/enable the Battery Charge Detection circuit that's built into this chips. 

Something like this:

PCD_HandleTypeDef *hpcd = (PCD_HandleTypeDef*)hUsbDeviceFS.pData;
USB_TypeDef *USBx = hpcd-&gt;Instance;

int stabilizationCounter = 0;

HAL_PCDEx_ActivateBCD(hpcd); 

//run on a 10ms Timer
if( USBx-&gt;BCDR &amp; USB_BCDR_DCDET ){
    stabilizationCounter++;
    if( stabilizationCounter &gt;= USBPC_STABILIZATION_TIME ){
        USBD_Start(&amp;hUsbDeviceFS);
        //stop the timer
    }
}else{
    stabilizationCounter = 0;
}


Note: HAL does have a void HAL_PCDEx_BCD_VBUSDetect(PCD_HandleTypeDef *hpcd)
 function, but your are expected to call this yourself after VBus power is detected. Which means an extra pin used and in my case a big layout reshuffle. Some details on the ST's recommended circuit is here USB hardware and PCB guidelines using STM32 MCUs

Is still have some 40uA caused by the LDO (~10uA) and a reverse leakage current of a Schottky diode (~30uA) but that's at least documented.