migo: no offence, but you probably didn't even ask a cop about it.
It is illegal to keep any weapon in a place concealed from plain view.
I think you can actually punished by imprisonment for no more than 10 years.
Go ask your local police station about it. (and make sure you don't keep any on you at the time, they'll test your knife too)
Any knife that could be opened centrifugally or by gravity or by a push of a button or lever or switch is considered illegal at the first place, and then if you keep a knife in your pocket instead of clipping it to your pocket, it's also illegal and they could seize and prosecute you for it.