Why were MI5 interested in my father during WW2?

My father, William Charles Loessl, as a man of no nationality,applied for British citizenship in 1935. Having discovered his naturalization document in the UK National Archive, I found them closed until 2043.
I eventually obtained them, by asking the Secretary of State to release them early, under the UK Freedom of Information Act. Their contents confirmed my fathers terrible plight as a child, during WW1 (as told in my book Mizpah Cousins: Life, love and perilous predicaments during the Great War era).

Some pages provide evidence that his naturalization documents have been seen by MI5. The MI5 stamps show my fathers papers requested out, then returned to the Aliens Department, on several occasions during WW2. Pa was serving in the Royal Army Catering Corps during this time.
I have discovered that MI5 are not subject to the Freedom of Information Act, so I am seeking a way to research this further.
If you have any advice on how I might investigate MI5's interest in my father, please let me know.
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