I have 4 cories in my tank. For a week or two after I introduced them into the tank, they used to come out when the lights were on. After that, I had to search for them and even then, I could see one or two. I tried multiple times to count them moving around my ornaments and rocks, ended up messing the layout.
I will suggest you this. As angellove78 said, throw a piece or zucchini or algae wafers but right before lights off. After 2-3 hours, turn on the flash on your phone, go near your tank and search for it. At least, that's how I found my cories. They were all sitting right near the middle, munching up on the algae wafers. I think I tried this at 1 AM or so. Good luck, hope Sergio is alive and just hiding from you.