When Ruppell's first appeared in Spain in 1997 I think many assumed records related to escapees from captivity. However, as records increased and reports of multiple sightings (5, 6, 7 birds?) began this idea was firmly scotched. It is now accepted that records refer to genuine vagrants from sub-saharan Africa. The lack of records from North Africa means little given the very thin coverage there. They've certainly been seen arriving exhausted from Africa along the south coast of Spain. Almost all of them are immature birds which probably reflects that in many species young birds are more prone to wander than adults rather than breeding. However, this can't be dismissed as a possibility as some adults are turning up and there were reportsd of a 'mixed' pair (Ruppell's x Griffon) in, I think, Portugal, some years back.
An equally interesting question is what this increase in records actually signifies. Does it reflect a real increase, greater observer awareness/skills or a bit of both? There's some suggestion that, with an historically high population of Griffons, the likelihood of the odd Ruppell's getting caught up with a flock of returning Griffons is now much greater than previously. On the other hand the increase in records mirrors an increase in reporting (largely due to the 'Migres' programme I think). I'm sure that back in the dark ages (pre-1980s) of international birding very few people would have heard of, let alone seen, Ruppell's and any odd 'dark Griffons' would have been dismissed as just that,