When did your network engineer start searching for a new job opportunity? When did he/she apply for a new job? I know the answer, and I will share it with you. It was Monday.
I know it because I was operating a job board for network engineers as a part of this web site. I was checking statistics for the efficiency and effectiveness of my campaigns. I realized that network-related job searches had a peak of 20% every Monday. It is a statistically meaningful difference, but it is not a big thing alone.
The surprising fact is that people click on offers at least %100 more than later days of the week. They are ready to accept new job offers twice as much as other days. It becomes 2.4 times when multiplied by the search tendency. The numbers are decreasing day by day and very calm on weekends. It means that Mondays have a diminishing effect on subsequent days.
So, what to do? You should arrange something for Monday mornings. There may be a one-on-one morning chat with each team member, a team breakfast, or a similar event(e.g., Taichi). Nevertheless, it should not be an official jour fixe. You should release your Monday mornings for the team. If you can have a good Monday with the team, you will have them for the rest of the week.
Happy Fridays are inefficient; we need happy Monday mornings!