To grow decent pot in a closet or any enclosed space you need ventilation, which in most cases involve an exhaust fan.
Anyone walking by the exhaust fan would be able to tell that something is growing in the closet.
If you for instance were able to vent directly up through the closet ceiling to the attic, they wouldn't be able to smell it from a sealed closet (but maybe outside when conditions were right).
carbon filter, buddy! im new to growin so there may be other ways..but i think its probably a great way. just google "homemade carbon filter" and im sure youll find lots of ways to build them. you can buy one too but they seem pretty easy to make (ive never made one. maybe its harder then it looks). but ive heard they work very well when the exhaust is blown through one. just thought id chime in...see what the others advise before you listen to me too much..good luck.
You can also search in google for neem oil(?) and neem oil depensers. I hear it works great to nulify the smell, but I've never used it. I'd check it out if your are worried about serious smell issues. Check out a product line called ONA. Made to kill odors.