The law appears to be this one[1], which bans secretly recording "wire" communications or "oral" communications, the latter specifically meaning speech. So it sounds like, in addition to secrecy, it's the audio that's at issue; a pure video recording wouldn't fall under this law even if done without knowledge or consent.
The opinion itself is here[2]. In fact the first two sentences of the opinion mention that expectation of privacy is irrelevant to this case:
> Massachusetts, like other states concerned about the threat to privacy that commercially available electronic eavesdropping devices pose, makes it a crime to record another person's words secretly and without consent. But, unlike other concerned states, Massachusetts does not recognize any exceptions based on whether that person has an expectation of privacy in what is recorded.
The opinion itself is here[2]. In fact the first two sentences of the opinion mention that expectation of privacy is irrelevant to this case:
> Massachusetts, like other states concerned about the threat to privacy that commercially available electronic eavesdropping devices pose, makes it a crime to record another person's words secretly and without consent. But, unlike other concerned states, Massachusetts does not recognize any exceptions based on whether that person has an expectation of privacy in what is recorded.
[1] https://malegislature.gov/Laws/GeneralLaws/PartIV/TitleI/Cha...
[2] http://media.ca1.uscourts.gov/pdf.opinions/19-1586P-01A.pdf