So i thought but no, he was there earlier, on a regular basis. So much so that by '75 for the movie PI he had to be asked by the director to go back to Brooklyn from Venice in order to create the impression that he was still in Brooklyn.
Lou says that he "couldn't train in California" for P.I., which I assume was for the movie as you said. But, I've not seen a single photo of Lou in Gold's Venice from the early to mid 1970s, nor heard any stories of Lou training there prior to 1976.
Just a guess though, whereas they were actually in the audience of the Mr. International to see Oliva. Most likely to see what he looked like prior to the Olympia. And if that interest was so for one year it was true for the next as well.
Unfortunately, I can't find any information on the 1974 WBBG Mr. International, which Sergio entered and effectively DQed himself from the Olympia, but I doubt Arnold was there in the audience, considering it was for a rival organisation. If Sergio was to compete, there would be no point of Arnold attending to check out Sergio, as it's an easy DQ. I guess we don't know without actual coverage of some sort.
He and Franco guestposed at the 1974 IFBB Mr. International, which Lou won, so it was a great time to scope out Lou if any. Lou was being talked about as the next poster-boy for the IFBB, so it's little wonder why Arnold wanted to prove himself as the top dog.