discreet way to lock closet?

I have a grow closet and I really don't want anyone to accidentally open it to find plants in there instead of clothes.

Is there something I can do to lock it or something? Just putting a giant lock on it is too conspicuous.

Does your grow closet have a door with a doorknob? If it does, you can replace the knob with one that locks. Don't get the bedroom door knob that locks by twisting a thingy; you want the one that locks with a key. It may look a little out of place, but nothing like a padlock.