What to look for from the ground
Compare the courses rather than looking only for a completely missing tile. A piece may have dropped a small amount, lifted at one corner or shifted sideways enough to reduce the overlap.
Why a tile moves
Broken nibs, failed clips, wind and impact can all loosen an individual tile. When access is available, the neighbouring pieces and the batten beneath should be checked rather than replacing the visible one and leaving an obvious second fault beside it.
Matching matters
A near match in colour is not enough if the profile or interlock is wrong. Forcing an unsuitable tile into place can lift adjacent pieces and create a leak that was not there before.