Red hot exhaust at iddle?

[Crissis]

Also posted this on different forums, with multiple answers. What do you think?

Hi,
Just dropped the keys on the floor when warming up the bike and picking them up saw how the exhaust was red hot all the way down to de O2 sensor

Went for a ride thinking it was the starter heating up the bike but on the way back, siting again at iddle for 1 min or so, the exhaust would become red hot again.

Is this normal? why only at iddle? (air flow or running very lean at iddle)?

thanks,