My question involves real estate located in the State of: California
My grandmother, who is 84 years old, bought a plot of land in Palmdale in the 70's for about $10k. Due to her advancing age and her irritation with paying property taxes, she ended up "selling" the plot of land to my parents for the same price she bought it for.
About half a year ago, the property title was officially handed over to my father. I thought it was pretty silly of him to buy the property without having looked at it first but hindsight is always 20/20. Since my parents live in another state, they asked me to drive over to the plot to scope it out. A week ago, I made the journey over to Palmdale from my house in Orange County. It turns out that there are two houses that have been built on their plot of land. I was pretty shocked since both of these houses appeared to have electricity and plumbing (I'm assuming). Call me naive, but I always envisioned squatters living in tents. One of the houses had a pair of dogs in their front yard that barked incessantly as I took pictures with my cell phone camera. Long story short, I sent those pictures to my father. He was incensed and started panicking but I reassured him that we would have to figure out our rights before moving forward with any sort of dispute.
I did a little bit of research on this, but I would like to get a second opinion on the next steps we should take. I do know that my grandmother paid property taxes every single year without missing a payment. Thanks in advance for any advice.

