Memory issue on server

Below are the memory details on one of my production server in our environment.

Total physical memory on server is 40GB

sql server is assigned 32GB

Amount of memory sql server is using is 28GB(total server memory from perfmon couters)

was able to see that availableMbytes has never dropped below 1000MB.

We are contantly receiving memory alerts on the server saying that physical memory on the server has reached 99.2%.

My first question is 

sql server is assigned 32GB and the amount of memory sql server is using is 28GB, which other process is using the remaining memory?

secondly 

is there a way to find which other process are using the remaining memory?

any one who can give me a clear understanding of this would be greatly appreciated.

Thanks in advance.