I reckon he's travelling around Europe and playing a pan flute
Seems the most logical solution so far... I certainly think I've seen him, now you mention it... ;-)
Actually Mikael is absolutely right, and we have Tintin's word for this!
In a strip, appearing only in the newspaper version on the 7th of July 1944, following a break in publication brought about by the war, Tintin sought to remind readers of the story so far by narrating the events up to the point the story broke off.
One of the points he makes is that "...au cours d'un violent orage, la foudre detruit la momie du Roi Rascar Capac, ne laissant intacts que ses bijoux..."
("...during a violent thunderstorm, lightning destroyed the mummy of King Rascar Capac, leaving nothing intact but his jewelry..."
This strip can now be found in the preface to Le Mystères des 7 Boules de Cristal
, the recent collection of the black and white newspaper strips
, edited and annotated by Philippe Goddin.
One must remember that the figure we see creeping about, looking in the window and throwing the crystal ball, is naught but a figment of Tintin's imagination, conjured up in a dream during the storm which eventually wakes him when the window blows open. It isn't the mummy walking.
On reflection, it is surprising that Rascar Capac hardly features in the book at all, given the long shadow he casts over its events: his mummy actually appears in a mere three frames, with a further five given over to Tintin's imaginary phantasm (and he's hardly in one of those - barely peeping over the sill of the bedroom window).