OS X 10.9 : How can Disk Utility activate my encrypted partition without a password ?

I don't know. The partition is on the startup volume. When I coldstart my machine and I log in I get asked for the password to the partition, which I haven't memorized yet. So I say cancel.

If I then go to Disk Utility click on the grayed out partition and "Activate" it, it just works without asking me for a password. I don't see an entry in the Keychain either and anyway, I made the encryption with the context menu of the finder. Weird and disturbing.

So I tried to reproduce this with an external USB drive, but there everything behaves as I would have expected it.

Anyway, here is a video showing it... I made it on a separate account, where it shouldn't have any access to any of my personal keychain information.

Update: Answer