This is a very narrative system where the players are placed on the scene and they develop it as they see fit but sometimes the GM sees and opportunity to spark something up. The book definition of a GM intrusion is as follows:
At any time, the GM can introduce an unexpected complication for a character. When he intrudes in this way, he must give that character 2 XP. That player, in turn, must immediately give one of those XP to another player and justify the gift (perhaps the other player had a good idea, told a joke, or performed an action that saved a life).
Often, the GM intrudes when a player attempts an action that should be an automatic success. However, the GM is free to intrude at other times. As a general rule, the GM should intrude at least once each session, but no more than once or twice each session per character.
Anytime the GM intrudes, the player can spend 1 XP to refuse the intrusion, though that also means she doesn’t get the 2 XP. If the player has no XP to spend, she can’t refuse. If a player rolls a 1 on a die, the GM can intrude without giving her any XP.
Being this a PbP, you ask for NPCs, that will not be considered a GM intrusion.
Now, if you are in a scene that you think GM should check out and perhaps add something to it then that would be an intrusion.
If GM sees a scene that they find interesting to post a twist, that will also be considered a GM intrusion. (note that you can spend XP to deny the GM intrusion and this will not offend the GM)