Place it at the reveal point in a spy or stealth edit, right after the action cuts to your entrance. Lead in with the pop sound effect for the appearance, play this line, then follow with enemy spotted when the other team finally notices you.
About this sound
The clip is the Spy from Team Fortress 2 delivering his short line 'Sorry to pop in' in his smooth, mock-polite voice, lasting about two seconds. It feels sly and cheekily apologetic, like an intruder who is not sorry at all, which makes the interruption land as a joke rather than an annoyance.