In twelve US states, not telling people is a crime. California, Connecticut, Florida, Illinois, Maryland, Massachusetts, Michigan, Montana, Nevada, New Hampshire, Pennsylvania, and Washington require all parties being recorded to consent.
That's for audio recordings I believe, and oh_sigh did say in another comment where legal in your jurisdiction
Yes, and then if you ever try to use those recordings it gets even creepier.
"I was afraid that people would make false claims against me, so I recorded my presence at some events. Check it out and you be the judge if accuser us telling the truth or not"
'sexual abuser records creepy video of his unknowing victims using a hidden body camera'