From a user point of view, booting a N9 has the following steps :
1) show animation
2) a little beep since PR_1.1
3) animation goes on
4) nokia tune
5) enter pin code
6) booted

First I have a question: I did not have step 2 initially. What does it mean ?

Second question: does anyone have insight about what really happens under the cover ? At least it must be checking for a flasher software on the usb, I guess. But there must be a ton of things happening... Any pointer ?

I used to have that little beep. I tracked the time of the beep and looked it up in the syslog and it seemed to be some assertion error about a strrstr and a haystack being null. I meant to investigate further but my beep no longer occurs.

The BIOS is accessible?

Humm... I did not activate developper mode, but looking at /var/log, I have a file named validator-init.stderr.
This file has a size of 0 bytes, and it is dated 01/01/1970 01:00:01.
Seems strange. Should I try to remove it ?