Well, I just checked the laws in my state, Massachusetts. This would be covered by G.L. c. 272, §99. The statute prohibits the interception of communications (without court authority or prior permission of all parties to the communication), -- "wire communications," defined at G.L. c. 272, §99.B.1 as
any communication made in whole or in part through the use of facilities for the transmission of communications by the aid of wire, cable, or other like connection between the point of origin and the point of reception
and "oral communications," defined at G.L. c. 272, §99.B.2 as
speech, except such speech as is transmitted over the public air waves by radio or other similar device.
So clearly they're looking to restrict recording spoken words. It seems to me that a surreptitious video recording without sound likely does not violate the statute. That said, this is not my primary area pf practice or specialty; and I recommend that you consult an attorney who does specialize in this before you undertake any activity to which criminal liability might attach.