Yes, there could be a planet lurking somewhere in the solar system that hasn't been found. There are things on Earth that have yet to be discovered, so it is quite possible that Planet NIne exists somewhere in the far outer solar system. Astronomers are searching for new planets everyday. If Planet NIne exists, they will find it.
In such an enormous solar system, there is no doubt additional planets that we have not discovered yet. It is only a matter of time before we locate them, as our search capabilities are continually enhanced by new and emerging technologies. Scientists already have some ideas of where to look based on gravitational pulls.
As large as our solar system is there could be a very good chance that there are still undiscovered planets out there. Our science is limited in so many ways and this can lead to smaller planets being hidden behind larger ones. The vastness of the universe is immense and to see all planets could be difficult.
Humans have done a good job exploring their neighborhood in space. Pluto was a planet for a while and then it wasn't. There are good astronauts and scientists that have explored space, and chances are, they have done a good job making sure there aren't any planets in the solar system that we don't know about yet.