Apple Snail & Assassin Snail?

I received 2 gorgeous, very big apple snails as a gift.
I put them in my tank and I just love them. But I know that ~somewhere~ in that tank there's an assassin snail. I put him there about a month ago. I have only seen him once, when I replaced all the decor. Crazy enough, he was out of the water for a good hour until I noticed him, and I thought I'd throw back him in just to see if maybe he was still alive, and the next time I looked, he was gone again. Dang!

I don't want that assassin snail to hurt these super cool guys, but I don't want to tear apart my decor to find him (which will churn up all sorts of crap in the rocks and cause a huge ammonia spike).

Help?
Any ideas?