Well, I live in a fairly small town with many small parks and no really big parks and have not felt any reason for concern detecting by myself. The small parks are easier to see what's going on in the whole area so that is an advantage.
Now no area is completely crime free so I do use common sense and stay alert, but I'm sure it's a lot safer than large parks in big cities. Also I am sure some cities have higher crime rates than other cities.
My usual travel range for detecting areas don't extend to the nearest big city so I have no experience yet with detecting in highly populated areas.
While I wouldn't mind having a hunting buddy to go detecting with I have not yet had one and have always had to detect by myself, and while my wife understands I have an interest in detecting, she has not wanted to tag along yet, of course she does have a busy schedule.
Again, as far as large cities, some parks could be very safe, and other parks might be in higher crime areas, you would need to research the crime rate for the areas you are considering.
I can appreciate what you are saying though, there might be some areas you might want to avoid feeling like some criminals might view a senior as an easy target in a high crime area.