Use a Mac as a security camera
A Mac already has the camera, the microphone and the disk a security camera needs. What it's missing is something that watches. OnCue does that from the menu bar: it records when something moves or makes a noise, and it's free.
What you need
A Mac running macOS 14 or later. About This Mac, in the Apple menu, says which version yours has.
A camera. The one built into a MacBook or an iMac works. So does a USB webcam, which is how a Mac without a camera gets one, and with two cameras OnCue watches both.
Somewhere to plug it in. There's no account to make and nothing to subscribe to.
Aim it
Open OnCue from the menu bar and click Preview. The window shows what the camera sees, with movement picked out in red. Move the Mac until the door or the desk is in the picture. If only part of the picture matters, drag a rectangle around it and OnCue ignores the rest.
Set how much it takes
The popover has a meter for motion and one for sound, each with a slider. A meter turns red when what it's picking up would start a recording. Walk through the room and move the slider until you set it off and a curtain doesn't.
Test it
Choose Record a Test from the menu. OnCue makes a short recording and shows you the file, so you know the whole chain works before you rely on it.
Arm it
Click Arm. It counts down first, so you have time to leave the room. From then on anything past a slider starts a recording, and each one begins a few seconds before whatever set it off.
Leaving it running
While it's armed, OnCue keeps the Mac awake. The screen can still go dark. On a laptop, leave the lid open: closing it puts the Mac to sleep unless it's plugged in with an external display, and OnCue warns you when that would stop it watching.
For a Mac that should watch every day, Settings can open OnCue at login and have it arm itself. It can also arm on a schedule, weekdays from nine to five for instance, or whenever a Focus turns on, so an iPhone that knows you've left home arms the Mac.
Recording takes about a megabyte a second for each camera. OnCue stops before the disk is full and says so. Give it a number of GB in Settings instead and it deletes its own oldest recordings to stay under that.
When you come back
Disarm, and the popover says how long it was armed and how many recordings it made. Click that line and Finder opens with those files selected. Every recording has a still beside it, taken at the moment it was triggered, so you can see what each one caught without opening any of them. Make a Time-lapse, in the menu, turns the whole stretch into one short movie.
What it doesn't do
OnCue doesn't send alerts, and there's no app for your phone. It has no network permission at all, and you can check that yourself. If you want recordings on another device, save them to a folder that syncs and they turn up there on their own.
Getting it
Download OnCue from the front page, or with Homebrew:
brew install --cask jvalol/tap/oncueEverything on this page, in more detail, is in the help.