The thing is, in this game you just don’t have to do this.
If a NPC is stealthy, PCs don’t detect it. You don’t have to tick them off or convey anything. Or you can say, when you want, that it is detected, if this makes the story progress. You give “tips” when you feel it’s OK for the PCs to act. If you don’t feel it’s appropriate, don’t give the tips. If the NPC is a Master at Stealth, it could jump on the PC without any warning. NOW the PCs are aware they were followed… but they have to resist or take level 3 harm or whatever.
Second solution: you “tick them off or convey” that a NPC is lurking in the background, but as a complication or a worse position, as a consequence of any roll.
In order to respect the fiction, the position should not always have been decided by the fact that there is a NPC (but it could be) . This complication is added to another consequence that reflects the position.
Example: a CONSORT is Risky because the target is not especially a friend and there is a risk they get angry by your dishonest proposition and hits you in the bo**ocks. On a 4/5, OK, you have convinced the target that you’re not that bad, but you realize someone has been spying your conversation… Maybe not the best example but you get the meaning.