In the film, there is a scene when Howard is trying to access a door to the ventilator room. That seems jammed, hence he sends Michelle through the duct.
ref : 10 Cloverfield Lane Explained
Who is Megan, Brittany?
Howard seems to have also used his bunker to abduct a girl a couple of years ago. Now this is only speculation
but there are sufficient clues that point to the abduction. Brittany
is the name of the girl he (allegedly) abducts. She is someone who
went missing a couple of years back. Howard carries a picture of
Brittany and claims that it is his daughter Megan.
ventilator shuts down, Michelle crawls over to the ventilation room
and turns it back on. Michelle finds Brittany’s earring in the bunker
and also sees “HELP” scratched onto one of the fiberglass doors. On
talking to Emmett, it is revealed that the picture that Howard is
carrying is not of his daughter Megan but a girl named Brittany.
Emmett confirms that Brittany was in the same highschool as his
younger sister. So looks like Howard has abducted Brittany after his
family moves away (and after he loses his real daughter Megan).
Many years ago, it looks like Howard kidnapped and held the girl (Brittany) captive in his bunker.