Loader pauses on Pi4 without input

A Pi4 running -current updated yesterday boots fine from a
USB hard disk, but for some reason pauses at the loader prompt
despite not getting any input on the serial console. There is a
monitor/keyboard connected, but the monitor is powered off and
the keyboard is out of reach, so I didn't bump it.

Is there a software switch somewhere that controls this
behavior? Eventually I'll want the machine to come up
hands-off, though for now there's no harm in typing the
boot command. 

Here's a snippet of serial console output:

Loading kernel...
/boot/kernel/kernel text=0x2a8 text=0x84ad60 text=0x247fd4 data=0x1b92a8 data=0x0+0x40d000 syms=[0x8+0x1331a0+0x8+0x15a6c1]
Loading configured modules...
/boot/kernel/filemon.ko text=0x1867 text=0x2558 data=0x510+0x20 syms=[0x8+0xd08+0x8+0x7c9]
/etc/hostid size=0x25
/boot/kernel/umodem.ko text=0x2100 text=0x13a0 data=0x6d8+0x10 syms=[0x8+0xf18+0x8+0xb5c]
loading required module 'ucom'
/boot/kernel/ucom.ko text=0x2590 text=0x2f00 data=0x8e0+0x858 syms=[0x8+0x1290+0x8+0xbd5]
/boot/entropy size=0x1000

Hit [Enter] to boot immediately, or any other key for command prompt.


Type '?' for a list of commands, 'help' for more detailed help.
OK boot
Using DTB provided by EFI at 0x7ef0000.
EFI framebuffer information:
addr, size     0x3e22c000, 0x8ca000
dimensions     1920 x 1200
stride         1920
masks          0x00ff0000, 0x0000ff00, 0x000000ff, 0xff000000


Thanks for reading, and any hints.

bob prohaska