I second this. Sometimes enemies will just stop and stare into space while one of them is decoy'd. Often upon cast of decoy the targeted enemy will direction turn its' attention towards you, repeatedly shooting at the place you cast the ability from if stealthed, as mentioned, while other nearby enemies do nothing.
If they do successfully attack the decoy, its' effectiveness will usually be a bit hampered by the enemies getting stunned by electric or heat procs and the like, but perhaps that is only fair.