There are the cases where the abductor is not someone who would be "missing" because they were never integrated into society to begin with.
Such as Elizabeth Smart's abductors.
There are also the cases where the victim is held in the suspect's basement, cellar, attic or elsewhere. Tied up, blindfolded, gagged, drugged, whatever. Then that person can go to work or do what they need to do.
Shawn and Ben's abductor left the apartment
Sarah Maynard was in the guys basement
Jaycee didn't interrupt Garrido's normal routine
The 62 year old woman who was abducted and her house burned down, was also being held at his house.
If you are holding the person hostage at your house... it is not suspicious when you leave and come back home. You aren't going to a cave in the mountains... you are just coming home. As long as you aren't buying anything obviously weird, you wouldn't draw attention.
Absolutely, it's possible Holly could be being held somewhere.
Nobody thought it was possible for Elizabeth to be abducted like she was... and then still be alive 9 months later.
I've learned to never think something isn't possible... it's always possible.
It may not happen, but it is certainly possible.