My father had an agreement with an acquaintance to park a couple vehicles on his property a few years ago. The property owner ended up getting foreclosed on, and he didn't tell my father. When we went to see if the cars were still there, months after the foreclosure, the cars were gone. Is there anything my father can do legally about this? Could the cars be considered stolen? For reference, this took place in Riverside County, California. Also, sorry if this is in the wrong law board, or of it shouldn't be in the law board...

You can sue the property owner (both old and new) for the value of the cars. You will have to provide proof that you owned the cars (or your father will) and the value of the cars. You may have to provide proof of the agreement to store the cars.

The person your father had an agreement with was obligate to warn your father to recover his property before the foreclosure. That is the grounds for your suit against him. At the very least he should have informed whoever took ownership of the property, that the cars were not his and that they should be returned to the owner. So the new owner had no right to dispose of the cars without determining who the owner was.

"You may have to provide proof of the agreement to store the cars". Absolutely! Depends on what the "Agreement was". "Yea, sure go ahead" or "let's sign this agreement". There is limited liability just letting someone park on your property. How much is "a few years"?

It will be SO EASY for him to lie his way out of this that I don't think it's worth court fees and time.
Example # 1: "I noticed at some time or another that they were gone and assumed that you took them."
That is a very reasonable assumption.
Example # 2: "I contacted you to come get them, but you didn't respond."
Also reasonable, regardless of how hard he did or didn't try. Change any contact info? The burden is on you.
I could go on and on....

All I can say is good luck... about 15 years ago I had a car at a body shop that went out of business and the property owner had everything towed. And it wasn't years (for bodywork and paint)... it was a span of 2 months. I never saw that car again. But then it wasn't worth a large amount at that moment,I was going to build it ground up as a street legal track car. So wasn't worth spending a lot to try to get it back... and nope, nobody tried to contact me before it was towed. But I was in VA not CA.