If a faculty member is considering leaving their department for another university, but isn't at all sure whether he/she will stay or go and doesn't want their department (other than a few close colleagues) to know about his/her potential departure and has grad students who would be stressed out (perhaps unecessarily) if they knew and who is in the middle of trying to recruit new graduate students.. what's the best way to deal with all that? Being completely open about everything doesn't seem like the best plan, but being secretive doesn't feel so great. Providing partial information seems pointless. It is important to note that this person would never abandon their grad students and researchers and would work out the best possible arrangement with lots of lead time (1 year at least) to organize things.
In reality of course, it's not up to the possibly-departing faculty member to control the flow of information because rumors fly anyway. So maybe it is best to be open with people who would be affected, if they are going to hear possibly unsettling information from other sources anyway.