I just recently subscribed to Navigraph and have seen many many airports now have that symbol next to them. They are just the stock airports, I haven’t done any mods to them.
As the legend shows, it stands for Third Party Content. In this case, I guess Navigraph is the Third Party and the Content they’re providing is behind-the-scenes data.
By the way, I’m really loving the features and functionality that Navigraph provides. Very very cool!