The technical debriefing has some answers for your question:

LOVELL: There was something  else venting,  too. 

SWIGERT: Yes, we had some venting from the CM periodically. LOVELL 
We  forgot 
to ment
ion  that . 
There was 
something 
all  during 
this 
period
,  while  we  were 
checking 
through 
the 
AOT ,  that  was 
vent
-
ing  out  the 
SM. 
We  att
ribu
ted 
the 
vent
ing  at  that 
time  to 
the 
hydrogen 
tank . 
SWIGERT 
What 
I  thought 
was 
happening 
was 
that  the 
hydrogen 
tanks 
would 
go  up ,  pop 
the 
relief valv
es ,  vent 
for 
a  perio
d  of  time ,  and 
then
,  the 
vent
ing  would 
stop . 
Jim  and 
I  were 
trying  to 
see 
whether 
we  could 
see 
stars  as  we  went 
arou
nd. 
We  found 
that 
during  peri
ods 
of  no  venting
,  there 
were  attitudes 
from 
which 
we  could 
pick  out 
whole 
constella
tions . 
Jim 
picked 
out 
Scorpio 
and 
Nunki. 
I  picked  out 
Acrux ,  the 
Alpha 
and 
Beta 
Cent
auri
, 
and 
the 
Southern 
Cross. 
We  could 
see whole 
conste
llations ,  but , 
when  the 
venting 
started,  it  was 
immedi
ately  apparent 
that   the 
stars  were 
gone
.             

.