Big fish eat little fish, and Pleco's, especially the so called "Common Pleco" as yours likely is; are NOT total vegetarians.

If starved for extra protein and offered something it COULD catch and eat, it probably did.

Is there any place the little one could be hiding? Inside a decoration?

That's a possibility too, because when put into a tank with a big fish, a little fish's instinct is to hide.

Plecos can grab onto a surface and if frightened, won't let go unless literally pulled away by force. So if you do have anything in the tank the little one could hide in, even if you lifted it to check the tank, it may have been inside, and not about to let loose.

We have checked all the ornaments and under all the gravel so I think he must have become dinner for the big guy:-(

I'm sorry for your loss, but it's how Nature made things.

However, if you have a large, protein starved Pleco, it's not going to stop with one of it's smaller cousins in it's quest for what it's body is craving.

Plecos don't really care what they get their suckers on. Most of the time they don't care. Most likely not intentional. I thought I had lost a fish more then once and found it at a later time thinking I've checked everywhere.

We had an algae eater...kind of a cousin to the Pleco??...well, we decided to get 2...get the job done in 1/2 the time!...it didn't work out that way...little "al" had a big bite taken out of him...an unfortunate demise. A reliable pet shop owner told me that they are very territorial and will kill another...I hope it's not your case here, but seems nature will take it's ugly course. I think I have only seen tanks with one Pleco each...I have one now that is non-stop in his quest for cleanliness...we have 4 tanks and 2 have Plecos...each in a separate tank. :)

I have two plecos in one tank...

Both common type, although one is an albino. They spent their first 3 years in separate tanks, though - then when I took down a tank, I put them in the big one together. They finally seemed to have slowed down in growth. The albino (the older one) is about 10 inches long, the common one about 8 inches.