Let $$\Omega $$
Ω
be a subdomain of a Stein surface with smooth strictly pseudoconvex boundary $$M=\partial \Omega $$
M
=
∂
Ω
. We are mainly interested in the case that $$\Omega $$
Ω
is not relatively compact. Building on work by G. Lupacciolu, we describe the envelope of holomorphy of an arbitrary open subset $$M^*$$
M
∗
of M in terms of a modified holomorphic hull of the complement $$A=M\backslash M^*$$
A
=
M
\
M
∗
, designed in order to also reflect the geometry of $$\Omega $$
Ω
at infinity. As a consequence, we clarify the relation between the envelope of $$M^*$$
M
∗
and some notions of core sets of $$\Omega $$
Ω
, which are an active topic in recent research.