This sign means that there is a shared connectivity between devices. Possibly network hub, or undiscovered switch which interconnects devices but stays invisible for NNMi.
Another possibility - if there are aggregated links and you are running on standard NNMi license, then this icon can appear itself as NNMi needs advanced license to correctly discover aggregated links.
Last but not least, do you use Cisco Discovery Protocol(or other vendor analogues) on affected devices? Do all of them use same protocol?