Video-only surveillance is legal in most states without telling you — audio recording is a different story.
Often, yes — for video-only cameras covering an open work area, no federal law requires your employer to tell you about it, and most states don't either. Courts generally treat video monitoring as legal if there's a legitimate business reason, like security or loss prevention, and it doesn't extend into places where employees have a genuine expectation of privacy, like bathrooms, locker rooms or changing areas — recording there is illegal essentially everywhere, notice or not.
Audio is a different and stricter question, because it's governed by wiretapping law, not just workplace privacy rules.
Under federal law, recording a conversation is legal with the consent of just one party to it — which could be the employer's own consent, as the one "party" installing the system. But about a dozen states, including California, require all parties to a conversation to consent before it can be recorded, and that applies at work too.
The core exception here is simply which state you're in — this is one of the most state-by-state areas of workplace law. Whether video-only monitoring requires any notice at all, whether audio needs one-party or all-party consent, and whether there's a specific electronic monitoring notice law layered on top, all depend entirely on your state. A workplace with locations in several states may follow different rules at each one. If a union represents you, your employer generally needs to negotiate with the union before introducing new surveillance.
Rules on audio and video consent vary significantly by state. Check your specific state's wiretapping and electronic monitoring laws before assuming either way.