Newbie questions: Door left open sensor, what brand/system?

Indeed it does. :sunglasses:

You’ll need an Aeotec WASH hub for about $140.

https://www.amazon.com/Aeotec-SmartThings-Gateway-Compatible-Assistant/dp/B08TWDNQ5Q/

Then you will have a choice of many different sensor options, probably at least a dozen different brands.

Zooz is a good reliable moderately priced brand that works well with smartthings. This is the house brand for the retailer The Smartest House, Who also happen to be a sales partner for the SmartThings Aeotec hub. They have very good technical support. And they make a nice simple open/close sensor which typically costs about $28 but it’s often on sale for a few dollars less.

But as I said, there are lots of other sensors that work with smartthings, and any open/close sensor that works with smartthings can do what you’ve described, so I’m sure other people will have other recommendations. Here’s a community thread to start:

FAQ: Door sensors? (2021)