Interview Attendances

When attending interview - what is the correct thing to do with current employers. Interviews are usually last minute arrangements that require a request for 1 day or an afternoon out the office at short notice.

Candidates in the past informed me that they were straight up with their employer that they were going for an interview and that this had caused the "cold shoulder" or worse. This raises an interesting question - do you prepare your employer for your eventual leaving them by admitting to looking for work elsewhere, thus attending interviews in agreement with them, or do you covertly attend interviews through taking last minute holiday in order to hand in your notice without anyone having seen it coming?