Does Craft have any default keys for flash messages? Does Craft have any default keys for flash messages like logging in or updating a profile?
Not particularly.

UserSessionService has setNotice() and setErrors(), which are just wrappers for Yii's setFlash() where we pass in an 'error' or 'notice' for the key.

Yii will take than and prepend the FLASH_KEY_PREFIX const which we set to Craft.UserSessionService.flash..

So an error's key will end up being Craft.UserSessionService.flash.error and a notice will be Craft.UserSessionService.flash.notice.