Possibly a health or fitness tracking notification. My Huawei shows a heart if I've done over 10,000 steps in a day.
As this is a real FAQ, and I've said this before, they really should stop using heart notifications for anything but real dating apps like Tindr, Ashley Madison, etc. as they frequently make people think and suspect the worse of their partners or fiances. Perhaps Google could put it in the Android developer UI guidelines, NO hearts unless its a dating app