I had to google "caltrops" since I've never heard of them before. o.o BUT that being said... I don't know if they are. Private residences and places where you have to pay to park? Like, the zoo and stuff? They have tire spikes so that you can't like, cheat and drive in there without paying. So I don't know if they're illegal.
...but um... that's weird for stuff to just appear in your house.