I've got 12v running up to my blinds, so I'd like to throw a couple of small cameras pointing outside, but hanging inside -- pointing out, and get a 'y' adapter to power the camera, connected to my BlueIris WiFi. Low cost 2Mpx would be ok. I'm mostly interested in seeing the weather when away. I'd like it to be fairly small too.

What kinds of trouble might i run into? Can it do motion detection through a window? Can it see out at night?
Any nice low cost options?

Hi,
You would need to check the current requirement for your chosen camera to see if the original power supply could cope with both devices in case they were operating at the same time. Worst issue would be IR reflection at night; you would have to turn the IR OFF as it would reflect straight back into the camera and swamp the image. I have a Foscam R2 (PT NOT Z) looking through a window with a paper disc covering the LEDs, as they didn't want to turn off
The smallest cameras are likely to be the Wyze cams with the RTSP firmware if you are OK to install that, but the V2's were flaky. I haven't kept up with the V3 RTSP development, but they could be an option due to their small size - although they work off 5V if I remember correctly. 12V to 5V dropper ?

For approximately $50 USD, you can get a capable camera and test it. If it doesn't work to your satisfaction, I'm sure you'll find a use for the camera and gain experience on what to look for. Below is one of the ones I have.

Amcrest New 1080p WiFi Camera Indoor, 2MP Pan/Tilt Home Security Camera, Auto-Tracking, Motion & Audio Detection, Privacy Mode, Enhanced Browser Compatibility, H.265, Two-Way Talk IP2M-841B-V3 (Black)


That's just an example of an inexpensive camera to try. I've not used it through a window.