If someone leaves your system for the same job in another (i.e. lateral move), that should get you thinking.
If the average new-employee retention is less two years before they move on to another position, you definitely want to take notice.
If your system sees people leave and then watches them flourish in another position, you shouldn't brag that "they started off in this system." It should raise questions as to why your system couldn't seem to hold on to him/her.
Employee retention has always been difficult in our profession but, sometimes, we unknownngly encourage people to leave.
The list below is a compilation of reasons I've heard Librarians give for leaving their positions. If any of these sounds like a familiar complaint of former employees, you may want to consider it, especially from the employee's perspective....
The reasons listed are ones I've heard before -- of course there are a few I've heard that aren't listed. So why have you all left positions?