Here's a fun new trick that Google just patched into Google Now, the company's card-based personal assistant: it can now keep track of where you parked. While there are plenty of apps out there that can help you remember your parking space, they all require you to open them and save your spot manually. In contrast, Google's parking tracker will save your parking location automatically. First noticed by Android Police, the new feature is part of Google Search 3.4, which is rolling out out to Android devices running 4.1 and above right now.

Google Now automatically detects your parking spot through Android's Activity Recognition system, a feature Google released at Google I/O 2013. Activity Recognition uses a mashup of GPS, Wi-Fi, cell tower location, compass, accelerometer, gyro, and barometer data to try and figure out what the user is doing. By using all the sensor data available to a smartphone, Activity Recognition can detect if the user is walking, driving, cycling, or sitting still, and it can trigger apps to do something when a change is detected. If Google Now detects that the user has gone from driving to walking, the car has most likely been parked, and pinging the GPS to save your location would be a good idea. All of this happens silently in the background without the user having to do anything.

While it might sound like a huge battery drain, preserving battery life is a priority for Activity Recognition. GPS is one of the biggest battery drains in a phone, and Google can do intelligent things like activate GPS if the accelerometer detects that the user is sitting still. The activity recognition comes from precomputed profiles, so the only CPU work the phone is doing is pattern matching (e.g., shaking up and down and going 15 mph is bicycling). If you have an Android device, Activity Recognition is already running on it, provided you opted into Google location services.

Like all parking location savers, this feature is for geared toward on-the-ground city parking or parking lots—Google still can't magically make GPS or Wi-Fi go through steel and concrete, so don't expect much if you're in a parking garage. iOS users of Google Now shouldn't hold their breath waiting for this feature, either. As we've established, it relies on Android's Activity Recognition APIs, which is something Google can only build when it has deep access to the platform.

So how do you use this new feature? Just park. Google Now will save your spot and have a card waiting for you on the walk back to the car.