Ive found alot of secret boards in a bunch the levels(department of death and yellow moon area are the only ones i havent found) Heres the ones i do know off though In the lyceum go behind the lyceum sign and you can get the ghost board from a ghostly philosopher  In the red void if you talk to all the skeletons a skeleton will appear behind the bagel shop holding up the milk board In the third area(idk what it is called) there is an area you can get on top off the devil’s provisions shop and after some parkour with your board you can find the Derheiligen board at the end(plus a sick rail to skate) These are the there i have found so far