Definitely the "belly button". Pics worked fine for me.
Here's an interesting video for info on them if you want to see more:
https://youtu.be/xOLuIfZo4ws
MarkL - age is irrelevant for this in my experience. I've got a 4+ year old male BP whose belly button is still evident and all of my others have it visible if you look in the right spot, though some are more obvious or noticable than on others.
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