I believe every planet to be a frontier of sorts just waiting to be explored.
Therefore, even Capital itself would yield its secrets very slowly. Yes, everyone knows about the Imperial Palace and Grounds. But, what lies hidden amongst the University grounds or lies beneath in vast array of sewers and waste disposal sites.
Universities have cadet and reserve programmes and are abundant with historical rumours. "IF", directed by Lindsay Anderson in 1969 portrays schoolboys finding a cache of WW2 weapons in their college and one can easily imagine a forgetful professor mislaying valuable or dangerous items.
Heating systems are upgraded over the years and in the case of Victorian buildings, they can be left intact, leaving heating tunnels which connect buildings and access or reveal subterranean caches or forgotten rooms. Harry Potter takes it to the extreme, but archaic academic institutions would have an abundance of redundant disused or forbidden architecture.
I come from Cheltenham, home of GCHQ ("Euro-Pentagon"), home of the first flight of the jet engine, home of a surviving Russian princess (hence the Faberge eggs in the local museum), home to many many secrets in the last 100 or so years. When you try to scientifically account for the abundance of royal, military and intelligence secrets in this "sleepy" rural town, then there are some clues. It is built on limestone, has soft water, is one of the first places of altitude that the Gulfstream hits, it's geographical location and topography all make sense as a "cold war" safe haven for the intelligence community, yet it is relatively close enough to the capital for commuting. There are some clues to the "sleepyness".
The unusual number of US military number plates, the strange road signs marking unmarked paths, the unusual aircraft, vehicles, patterns all are big clues that it is not just about GCHQ, but these facts are not readily available, until you actually visit and even then you would have to know a little or care a little to discover these secrets. It is aided by the disposition of the "sleepy" locals, situated in the "mild mild West", the location is generally affluent, conservative, traditional and establishment-oriented. In other words, it is the ideal location to site a crucially important security or defence project or to accomodate a VIP in some safety. Gloucestershire once had the highest concentration of US forces anywhere in Europe, but driving through you wouldn't know it, except for the occasional space shuttle/747 or lost B-52 heading for Fairford. It's also relatively close to Bristol and Boscombe Down and also the SAS HQ and Marine HQ. In fact very little is in the capital, but these sites are all relatively close. Call Wales the Solomani and Scotland the Spinward Marches.
Personally I like the Spinward Marches and anything rimward, but also interested in Core/Solomani and 2300.