ninja fish

I met someone before who had a 100+ gal tank who had an adult syno angelicus they hardly ever saw.
If you have enough hidey places that you never remove and a fish that hides most of the time, it happens.
 
Heh, that reminds me. I was at the lfs the other day looking at kribs, and there was this thing I assumed was a rock. I didn't really look at it and didn't pay any attention. Then all of a sudden I realized it was a 14" pleco! So, yeah, they're good at hiding, lol. And yeah, it looks like you've got a pretty big tank and lots of hiding spots, so it isn't unreasonable that you didn't see your plec for 6 mo. Heck, I loose my 2 L387s in their 16g tank. And it's a tall tank, so there really isn't much bottom space for them to be in. But, there are lots of hiding spots. Don't worry, btw, mine are moving into their new 75g home soon. They are only about 2" each right now.
 
Give em places to hide and they will! That pleco had tons of places to hide from you! The fact that he grew approximately 4" in 6 months time would seem to indicate that he's happy and well fed!
 
From the picture of the rocks in your tank, I can see where the pleco could find places to hide.

My pleco likes hiding inside my water castle decoration.
 
AquariaCentral.com