When it comes to trying to find places affected by preturnatural energies, a good place to start is any location never meant to see the light of day. Hidden rooms are almost always associated with activities that are either nefarious or simply distasteful to the wider public, and are thus infused with the complex memories and feelings that are synonymous with lingering spirits.
Alternatively, you could get lucky and find a place that is not only ripe with the history that attracts wandering souls, but perhaps even a place dedicated to the practice of ancient rites and arcane pursuits. Of course it should be assumed that a location of that kind would be much more likely to hold dangerous artifacts or perhaps even traps designed to deter any would-be intrduers. Because of this, and of course the normal amount of risk that comes along with exploring any unmaintained building, caution must be exercised, and whenever possible an expert on the location should tag along as a guide.

A trapdoor in an abandoned house that led to a basic fallout shelter.

Many ancient and eldritch forces exist deep in the earth, so the deeper you can get, the more likely you'll be able to encounter paranormal phenomena. Not everyone has access to optimal locations like ancient crypts and abandoned mines, but most people have drainage tunnels nearby to their homes!
While these tunnels seem mundane, they still succeed in getting you deeper into the earth, where old things dwell, and in addition to that, they sometimes find themselves home to dark dealings, which is yet more potential for lingering spirits. Because of this, it is not unheard of for many towns to have their own local legends about nearby drainage tunnels, such as the Gates of hell in New Jersey.

A waterproof flashlight is recommended for exploring flooded tunnels like this.

While unusual or notable plant growth can be perfectly innocent, it can also be the sign of supernatural influence. One example that many people may be familiar with are fairy rings, a colloquial term for a patch of mushrooms or flowers that grow in a perfect ring. These often represent a place to commune with otherworldly beings or even where a portal into the dreamworld can be found.
The exact effects of each instance depends on what exactly caused the unnatural growth. Some examples could have been formed by mortal spellcasters, seeking to speak with forces beyond our realm, or by those forces themselves. Research on individual instances should be done to determine the source of the change, as approaching these phenomena without an idea of what to expect can leave you unprepared in a new and dangerous location, or conversing with something you'd rather not talk to.

An arch formed of dying trees, leading to an unnatural darkness.