DD3 has a birthmark on her inner thigh. Just a normal brown one that is growing with her, but when she was tiny it was right in the crease of her thigh & I honestly though it was a poo stain. I tried so hard to clean it off...
Ds2 has a normal brown one on his calf and when he was little I always thought a bit of poo has come out his nappy! Now he's up and walking you can notice it even more but it clearly looks like a birth mark.

My second cousin had one like you are describing but it was in the middle of her forehead, she's now 5 and it is completely gone but they did have a lot of people ask about it