How do people manage other factions running a job on the PCs?
Does it happen off-camera, with the PCs just hearing about the aftermath or… is it some kind of reverse score? Mechanically speaking, how do they defend against this kind of thing? How should it be interpreted in the macro-rules of either running a score/spending downtime?
One way it has happened a lot in my games is through the entanglement roll. That’s where friends want favors and foes want a piece of territory. Then that gets interpreted so it can be a payoff, or it happened and now you get revenge, or it becomes a mini-heist in a defensive direction. My main guidance for that is, watch how players react. If they shrug and find it annoying, let them pay it off or take the hit. If they get their hackles up, give them a chance to fend it off, or let them do a counter-hit, and switch to heist tools.
Another option is to put out a clock that, when it is full, a faction takes a crack at them. Let them see who it is or not. Fill one segment per down time. Let them do a downtime action to figure out who is after them, and then use downtime actions to take segments off the countdown clock.
Another option is to keep this aggression in mind, and then bring it in as a complication at the beginning of a heist where it interferes with the job, or let them find out their target is reinforced and forewarned because of the rival, or basically use them to gum up the works any time there’s a complication. =)