|From Whiz Comics #11 - Cape-Less Captain Marvel!|
No, Sivana didn't take it, nor was it part of the story line in each comic. Interestingly, Cap was cape-less for legal reasons stemming from the lawsuit between Fawcett Comics and National Comics which published Superman.
|From Whiz Comics #12|
On page 43 of The Fawcett Companion it states that "...Captain Marvel is deliberately shown capeless as an experimental concession to DC to eliminate similarities to Superman. However, when Fawcett’s lawyers discovered Superman’s defective copyright technicality, Captain Marvel’s distinctive cape returned to him after disappearing for only two issues (Whiz Comics #11 and 12)." (Fawcett Companion, ed. P.C. Hamerlinck (Raleigh, NC: TwoMorrows Publishing, 2001))
I'm glad they kept Captain Marvel's cape as it is an integral part of his uniform. His cape is just as important and significant as the lightning bolt on his chest and captain's rank on his sleeves!! While Captain Marvel's cape-less caper lasted only two editions, ultimately National Comics/Superman would win the legal battle.