Greetings Nebula,
A "sense" is an interface which coveys information about the physical world to our brain. With the use of technology we are able to extend the number of senses we were born with. A radio extends our sense to include the electomagnetic spectrum below the infra-red. We have transducers that extend our senses above the ultra-violet as well; into the spectrum of x-ray and gamma radiation. A search is underway that we might be able to sense gravity waves, etc. Work is also taking place with the aim of bypassing the usual five senses, by directly stimulating our brain with electrical impulses. The spectrum and sensitivity of our senses is growing rapidly through technology.
Nebula, I don't share your idea that we are endowed at birth with as-yet, unknown senses. A mechanism that allows us to sense things in the physical world must have at one foot inside the physical world it senses. In our case, the implication is that at least part of the mechanism is made of meat, or at least connects with meat. By now we've made exhaustive dissections and scans of the human brain. There don't appear to be too many left-over pieces.
Researcher's follow up nearly every credible lead. For example, there have been suggestions that we might have a compass built-in our heads in the form of magnetic iron particles. It came as little surprise to me that experiments failed to prove that humans have a workable internal magnetic compass. If you spun me around three times inside a carboard box, I'd have no clue which way is North. Yet, if I had time to think about it, I could probably fashion a compass from a bit of metal in my belt buckle that would align with the earth's magnetic lines-of-flux. That is, I could build a device to create, in effect, a sixth sense.
Cactus Jack made the excellent observation that a sense is only useful if it delivers quality sensory information to our brain. If a Hippopotamus is charging towards me, my senses would be worth precious little if they only gave me a fuzzy, or nebulous feeling that I might be in some sort of danger.
In reply to a friend's suggestion, I performed an unscientific "experiment" last winter. One quiet evening as my wife and I were reading, without noticably looking up from my book I began to mentally scream her name. I did everything I could think of to "quietly" attract her attention. I kept this up for nearly half of a minute. She never as much as glanced away from her book. My friend had suggested that since my wife and I have spent a good deal of our last 25 years together, that we might have developed another form of communication. My simple "experiment" indicates that if we have formed another method of communication, it's worth "a whole lot of nothing."
The unconscious mind is the power-house computer in our head. Yet it only provides our conscious mind with what it decides is useful to us. The information we are conscious of has been drastically filtered. Our unconscious mind knows far more about the world than it ever bothers to tell our conscious mind about. I believe this is the reason for our occasional "spooky feeling" that we know more than we ought to know. I believe the mind is a vastly complex piece of equipment, whose inner-workings will ultimately be explained in quite simple terms. We've already made fantastic progress in this direction.
So, I don't think you're crazy either, Nebula. Your question shows that you are thinking.
Michael