How to enhance your auditory sensitivity

We all experience it sometimes. You take a day or two (or six) off from your instrument, and when you start playing again, you actually have a feeling it sounds even better than it did before, even though you aren’t in shape. But is this in fact an illusion? Does our ability to hear small details get worse when we take a break? And if so, is there a way to train our ears as to hear out of tune playing, rusty attacks, noise between bow changes, etc, more clearly?