Certainly awareness of your performance, your relationship with your manager, what's happening with the company/your project etc. all may send signals that it's time to start looking elsewhere. But sometimes executive decisions happen without a lot of warning. And sometimes, to the point of a lot of the discussion here, something may be affecting the sector as a whole (or wider) so it may not be easy to find another position. A lot of people who post here seem to assume that getting a new job is something you send a few emails about and expect to get deluged with interview offers. That's not true of many people even today and certainly isn't true at all during downturns.