My experience at the California wasn't looking for slot cards, but only their Silver Strike machine. It was the first time I had been there and was roaming around, back and forth, looking for their machine, when lo and behold, I got the same treatment you described. I felt like a criminal. I had just bought $100 worth of strikes at one of their "sister" casinos, but after that incident I swore off collecting any tokens from any of their casinos.
BTW, that is the only time I've ever been "hassled" Silver Strike or slot card hunting.