How can they be missing?

I have been going through stuff getting ready to have a garage sale.  I was going to have it starting tomorrow but since I have my CT Angiogram, that’s not going to happen now.  Good thing I didn’t place my ad for the sale in the paper yet.  So I found our old digital camera but the memory cards are missing.  I had two memory cards for that and there are some photos on the one I wanted and you think I can find them?  I know they were in the case with the camera, so how could they just come up missing?  I checked every where.  I know my stepson borrowed the camera when he was putting stuff he wanted to sell on EBay, I think that is where they are, but he swears up and down he doesn’t have them that he returned them.  So now I am driving myself buggy trying to find them.  Don’t you just hate that, you know you put something some place and then poof missing?