Edges take the pressure
Verges, ridges, eaves and projecting leadwork are exposed. A loose edge can start moving and then pull adjacent material with it.
Slates can move without falling
A slate that has slipped only slightly may leave enough gap for rain to enter. From the street it can still look almost normal, especially on a dark wet roof.
Listen as well as look
Rattling in the loft during wind can indicate something moving, though it is not always roof covering. Loose vents, aerial fixings and guttering can make similar noises.
Repeated wind damage
If the same patch keeps failing, I would want the fixing method and surrounding battens checked rather than simply replacing the visible piece again.