STM32F4DISCOVERY high power consumption in Standby mode
Turns out the extra consumption was due to resistor R34 (pull-up for BOOT1). The user manual incorrectly claims SB19 shouldn't be connected at manufacturing, which wouldn't incur any extra power consumption. However, my board had it, as well as another, newer, board (part number MB997D, which is now called STM32F407G-DISC1).

Removing this resistor brought power consumption down to 3 uA.