Yes. BACtrack Skyn is collecting data at all times when the device is powered on. The Skyn hardware can store approximately 72 hours worth of data. As long as you turn on your Bluetooth and appropriately connect your Skyn with your app, the data will still sync and upload. But, if after 72 hours you do not connect to your app, the oldest data points will be overwritten by the new data coming in.
Please note that you are only able to see the last 24 hours worth of data at any given time.