@Hicdraconis
There are many many reasons why someone may not want their DP reading their hospital notes. We document previous medical history thoroughly, which may include things they don’t want their DP to know about (the most obvious being previous abortions and mental health admissions).
But it doesn’t matter if it’s a one page clerking of PMHx - nil, DHx - nil, Allergies - nil or a 5 page essay of ICU admissions - it is confidential to the patient and your DP had no right at all to pick it up and read it. Patients can request their notes but it usually has to be done in the presence of someone who can explain any medical terminology.
I would have taken the folder from him if I had been there and told him they were your confidential medical notes and he wasn’t permitted to read them. More or less politely depending on his reaction. I’d also have mentally flagged you (quietly) for a chat later to make sure there was nothing else untoward, like domestic abuse.
All of this.
It's got sod-all to do with 'trust' or not being a close couple, and it really fucks me off that people make it about this.
Some people prefer some things to remain private and confidential, and YES - even from their partner or spouse. I don't know why some people have to tell their partner every last bloody thing about themselves, from their personal and intimate medical details, to the size of the boil on their anus.
It's absolute shite to say you're not a close couple if your don't share everything. There are several personal things my husband doesn't know about me, and never will. And there doesn't have to be 'issues in your marriage,' or domestic abuse going on for someone to want to keep certain things private.
Keeping certain things to myself, doesn't mean me and my husband are not close, and the judgy posters suggesting this can bore off. 
We are a married couple, but we are also individual people with individual personalities.
And the ones who are saying 'me and MY man tell each other everything;' dream on, no WAY does he tell you everything; NO. WAY. 