  A sequence in a $C^*$-algebra $A$ is called completely Sidon if its span in
$A$ is completely isomorphic to the operator space version of the space
$\ell_1$ (i.e. $\ell_1$ equipped with its maximal operator space structure).
The latter can also be described as the span of the free unitary generators in
the (full) $C^*$-algebra of the free group $\F_\infty$ with countably
infinitely many generators. Our main result is a generalization to this context
of Drury's classical theorem stating that Sidon sets are stable under finite
unions. In the particular case when $A=C^*(G)$ the (maximal) $C^*$-algebra of a
discrete group $G$, we recover the non-commutative (operator space) version of
Drury's theorem that we recently proved. We also give several non-commutative
generalizations of our recent work on uniformly bounded orthonormal systems to
the case of von Neumann algebras equipped with normal faithful tracial states.
