Ive been searching for this for a while

Could it maybe be, oh, I don't know... "Mission Complete"? Or "Mission Beat"?

If you mean after the chapter is over in single-player mode, the track is "With You" by Lita Harvey.

However, if you're talking about like right after a mission is cleared (online or offline), well, it's not on the OST, so you're out of luck.