You asked "why would someone hide prep", you got one example. It is by no means an exhaustive list. For example, a big fear is being perceived as having HIV (since prep drugs are also part of HIV treatment).
If you do actually have interest in this topic you could read about it:
"Men were able to initiate PrEP without discussing it with their partners, whereas some women said they needed to get permission. Discussions around starting PrEP could raise questions about trust and infidelity and act as a barrier to PrEP use."
If you do actually have interest in this topic you could read about it:
https://www.aidsmap.com/news/feb-2020/your-partner-not-angel...
"Men were able to initiate PrEP without discussing it with their partners, whereas some women said they needed to get permission. Discussions around starting PrEP could raise questions about trust and infidelity and act as a barrier to PrEP use."