As far as I know, they do not randomly check people visiting ANY theme park. Or the areas around, or, at all. Right? I think it's illegal. There is a check point, which I believe is used for emergencies and to weigh trucks, I think it's in San Onofre? Or just before? If you see them around, they may be answering a call, or, just cruising around.
There is no checkpoint near or on the way to Sea World, but when we left the park there was a border patrol truck, I did not notice that they searched anyone but there it was on Thursday, January 3, 2019 that we went.
I have not seen any immigration near or in sea world
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