The heist and the anti-heist! It is reasonable to have factions fill up clocks, and when those clocks are full, to hit crews with the consequences.
In a less technical direction, I have found another way to determine when the crew is the target of action rather than acting.
When they are driving forward and pursuing a goal with gusto, I tend to go along with that and follow the action, only interjecting when it is the most fun. (And that’s where full clocks become a worry, that they can interrupt things and heighten the suspense and tension.)
When they lose direction and purpose and don’t have any clear idea for heists, or next steps, THAT is when I tend to hit them with NPC factions acting out.
They weren’t sure what their next steps would be, but after the assassins their enemy hired fail to kill them, you better believe they’ve got a pretty clear idea of what to do next. =)
Also, the moment when they’re on the edge of getting what they’ve been working for is also the prime moment for them to be targeted for a heist.