It’s also very noticeable and easy for someone to steal.
Your helmet is also open to the elements, although by being the right way up, at least it won’t collect rain like it does when on a helmet lock. Just remember to close the visor!
If you do store it on the seat, the most secure way so it won’t roll off is to have the visor facing the side with the sidestand.
Like the seat, it depends on the bike and is open to the same problems of security.
Hang it on the handlebars
The quickest and easiest place to hang a helmet is on the handlebars.
This can be secure, especially if you can get it over bar-end mirrors to stop it slipping off.
However, this can still be a precarious position where the helmet can easily fall if the bike is bumped.
It could also compresses the interior foam liner if you hang it with the helmet facing down. Compressing the liner can make your helmet fit more loosely and decreases its ability to protect your head in a crash.