Swap space alert during the off peak hours ?

Folks,

I get this email below from my Symantec Messaging Gateway v10.5.1 intermittently during off-peak hours:

The swap space utilization on the following hosts has reached the threshold (60%): 

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Host                     Swap used                Swap free                Date

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Local Host               2,527,784 KB (60%)       1,714,984 KB (40%)       Sun Dec 21 20:35:01 EST 2014

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Why that happening during the non-business hours?

the VM specification is 4x vCPU and 4 GB of vRAM

VMware vSphere console show the average weekly usage pattern is averaging 38% and the maximum is 43% and anything Swapped In/Out are all 0

So, clearly there is no problem at the Hypervisor layer or VM specification.