Daughter referred - Residency status verification

[Scantilydoesit]

My daughter's GP referred her to our local hospital for treatment a while ago.
This morning, I got a letter from the overseas visitors department asking me to send in documents to verify her residency status in the UK. She was born in the UK and has never lived anywhere else.
I was pretty surprised as her father and I are both British, were both born and raised here and my daughter has had several admittances to hospital in the past. (albeit a different hospital).

My husband and I are separated but he has an African name which we both still use. ( our dcs are mixed race).

I can't imagine how she cannot be found in the system through her records. Surely her medical records would show her birth, admittances and discharges of past visits?

Do they just randomly pick people out to check on ? I just can't understand it.