nuffsed said:

Any ideas what this symbol actually means? its obviously a warning but a waring about what?

http://prntscr.com/kofkj9

Any idea? 

It means that you have a room to produce new traps, that exclamation mark appears either after you upgrade one of agreements that allow you to produce more traps or when your city was attacked and you have to rebuild traps that were destroyed.

Misotisa  is correct, its  to let you  know you can produce more traps 

thanks Misotisa  :)