I'm running 14.2-RELEASE on a tiny Wyse 3040 thin client and it had the annoying habit of not booting unless a monitor was plugged in. FreeBSD was the only OS where this happened. Even if you plugged the monitor in after power up it still displayed nothing and would not boot. I know they make dummy plugs to solve this but it was definitely a FreeBSD problem. After lots of searching I found out that making changes to /boot/device.hints fixed the problem. Take out the existing hint.uart lines and replace them with these two.
Now it boots up fine without a monitor.
Code:
hint.uart.0.disabled="1"
hint.uart.1.disabled="1"
Now it boots up fine without a monitor.