but police department admits error and returns the sign:
The officer misinterpreted the sign as threatening, said Capt. Steve McCool, of the Oklahoma City Police Department, and took the sign, which read "Abort Obama, not the unborn."
Chip Harrison [the driver of the car from which the sign was removed] said his sign was to be interpreted as saying something like: Remove Obama from office, not unborn babies from the womb.
The [police] officers [initially] confiscated Harrison's sign and gave him a slip of paper that stated he was part of an investigation....
A bad decision on the officers' part, but a correct one on the higher-ups'.
The Secret Service also apparently visited Harrison and asked "to (walk through the house) and make sure [the driver] wasn't a part of any hate groups" (I quote Harrison here). "He said they interviewed him for about 30 minutes and then left, not finding any evidence Harrison was a threat to the president." This seems a bit heavy-handed.
At the same time, law enforcement is indeed entitled to investigate — and to ask people's consent for searches — based on nothing more than a hunch, or a sense that there's a very low probability that the subject of the investigation may have committed a crime or may be a planning to commit a crime. And such a hunch or felt probability might be based on what the subject is saying; if the statement is ambiguous, they may investigate to resolve the ambiguity. (I assume that the Secret Service was interested in whether Harrison belonged to groups that might be a threat to the President, not to "hate groups" in the more general sense.) I don't think there was any real ambiguity here, but the Secret Service is naturally and understandably pretty careful about such things.
Thanks to Prof. Jared Williams for the pointer.