If you need something to remotely sense something, like if your “mom is coming,” there are many ways to do this. One such way can be found on this instructables page, using an ESP8266 chip to communicate via a WiFi mesh network. The idea behind the project is that a sensor can be plugged into one end of the network, and relay this information to where it’s really needed. An ultrasonic sensor is used in that example, but it could be expanded to take advantage of different sensors or output devices.
Though the instructions seem to outline a 1-hop setup, the range of the system could theoretically be expanded to up to 5-hops. This would mean that you cold not only detect that “mom is coming,” but perhaps that the door of a nearby Starbucks just opened, or that someone is working out on your favorite treadmill in your apartment complex!
Most of the components for this particular build come from the Travis Lin Store, which features various products based on the ESP8266 module, as well as tiny screens to use with them.