That's what I figure, too; although I wouldn't have considered it in the past (in other words my pre-BB Board life!!).

It would appear a pretty bold move of Fredo to accomplish something like that practically under Michael's nose. And especially after the guilt pangs he seemed to suffer that very afternoon while having a drink with Michael. But by then he was well into Roth's pocket so may have felt obligated to put out the warning...and besides there's no other explanation I can think of as to how the police would've been notified.

UNLESS...after the ambulance took Roth from the hotel someone may have discovered Ola's body, thereby indicating foul play and an impending attempt on Roth's life as well. That alone would alert the militia without any help from Fredo Corleone.

Apple

ps - Also, at the party when Mike asks him where he's going, Fredo seems a bit too relaxed & ambivalent to be on his way to put out a warning for Hyman Roth. Could be though, that he's had enough to drink to be able to carry it off.