[danieltillett]

This is more a meta-answer, but it is a pity that more companies (and employees) can’t be professional about this. Every employee should feel comfortable having a new potential employers talk to their current employer. If everyone was upfront and honest the workplace would be a lot more productive.

One thing I do with my employees is have a chat to them about making sure they let me know in advance if they are thinking of leaving and in return I promise to provide them with a fantastic reference.

We all work in a open office so every employee gets to hear my references of past employees so they know how this system works. This approach stops me being sideswiped by employees leaving at short notice and my ex-employees get great references. A great reference from your last (or current) employer is worth its weight in gold.

[zerr]

Notice happens after the offer is on the table. Otherwise, you can interpret various signals such as refusing a raise, career progression is not aligned with the expectations of an employee, etc...