So it's along with the battery, clock, and screenshot icons. Which likely means it's from some system process or app. It's something that the manufacturer or carrier put on the phone, rather than some user installed third-party app. If you know what phone it is, make and model, the user guide might tell you what it's various notification icons mean
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