I made a walkthrough for ALL of the hidden crests.
Who cares about reading, watching videos is much more fun! =)
There are 125 hidden crests, but you can also earn 25 crests by collecting mini-crests. (25.000 mini crests for 25 big ones)
When you see a 'bubble' use Depulso (flick the right joystick 2 times forward) and mini crests will fall out. Walk over them to pick it up.
So:
25 Crests by collecting mini-crests
125 Crests by finding the hidden ones. (10 of those you get after completing a mini-quest near a Lion statue)
=
150 Crests = Crest Collector 14 achievement Unlocked
EDIT: People have been claiming I didn't show every crest. Well that isn't true, and to prove that I made a screenshot of every crest in my video's.
Put all of those screens in 1 folder, and this is what you get:
http://dullenl.wordpress.com/2011/04/30/harry-potter-and-the...125 screenshots for 125 crests. Hurray, so I DID show all crests! =)
Now stop bugging me :P
Update 4-10-2011:
Fixed the link to the screenshot and added 1 more video (the last one). You will probably not need it, because those 3 crests are almost impossible to miss. But for the sake of having them all on 1 page I decided to post it anyway. Weird I didn't do that sooner.
The screenshot link goes to my crappy blog. You don't need to click around on it, I'm not trying to promote it. But if I used a link directly to the image it would show in this solution. Nobody wants a huge image here, right?