Can you press that button and see whether it is working. You can also AC power cycle the server and check the behavior also.  Can you also check the rear side and see whether identification LED working on the rear. If none of these working then it could be a H/W issue with ID button.
Hi, we have four r740, the system health and system id led on the front (below the letter i) of one server is not lighting today, and the other lights of it are still on, including the power light.  Our other r740 are still fine, the system id led is solid blue.  I wonder is there anything wrong and why this is happening?  Thx [IMAGE]  
dell r740 system id led not lighting