You found the 1 hidden thing on the EFFL website. Trust me, there are no more hidden objects, so don't waste your time looking for them.
If anyone can name what part of the body this face-like cross-section was taken from, I'll put your name on the website.
I'll give you two hints:
1. It has a high capacity to fill with blood.
2. It is the part of the body most obviously effected by the drug sildenafil.