Calling on the [Willing suspension of disbelief](http://tvtropes.org/pmwiki/pmwiki.php/Main/WillingSuspensionOfDisbelief) trope.

I think it was just an overlooked detail when it comes down to it.

To add on to this, as I recently reread Buddha, there are plenty of other spots where they should have noticed his brand. Like the time he was hung upside down after bragging during training. There really is no explanation. Maybe he came up with some way to mask it, like body paint, or something.