Can I walk on someone's property and pet their cats?

There is no sign or anything. Its near a park where I play disc golf and Its not far from the tee of one of the rounds. I don't do it when they are home.

If I ever saw the owner I would ask to make sure its ok but I never do and I don't want to knock as it would be imposing.
Update: Door to door salesman can bother us without "trespassing" but petting a cat is?
Update 2: I will try to ask permission if I see her. I think its a her. Could be a him. I saw some older folks knocking on the door once. Whoever it is, they are not home at 6-7 am but they are at 6-7 pm generally. I play one or the other in summer.
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