Some of the nonsense on this thread is rather alarming.
What your dad should do is:
Take a picture of the room and entrance and put the mirror back when it was so no one realises he found that space
then speak to the police straight away and let them deal with this.
Of course it could be dodgy as hell and needs to be reported...
I can't believe the number of people who are trying to say that this should just be left alone and that it could be a bomb shelter or a teen hideout.
It is as likely that it could also be used for criminal activities: someone hiding there who might be on the run; someone who is in the country illegally is being exploited or something even more sinister.
The point it is not for your dad to investigate and decide on his own.
He needs to report it.
And he certainly should not raise it first with the owner because again you have no idea as to whether this person is just a regular old guy with a random room in his basement or a complete creep who is part of criminal activities.
As for the person who went on about respecting the old man's privacy. Have a word with yourself....
Whatever happened to common sense these days.