Indicator in mode-line

[Heime]

I want to issue some indicator to the mode-line when some condition happens 
(error, 
warning, notification).

I can add things to the mode-line is the following way.  
 
(setq mode-line-format (list "Indc " mode-line-format))
(force-mode-line-update)

But also require a way for the user to acknowledge the indicator
and be able to switch the indicator off - returning the mode-line 
as original.

What kind of indications would work well in getting user attention?