Why was he even there? Well, depends on which there you meant. Disneyland itself--Frodo spent at least one day in the park every month, which of course included an overnight stay at one of their three fine hotels. (It was a childhood tradition that maybe should have ended by now.) But if you meant the rooftop--well, that was another story, albeit a simple one.
He'd climbed to the roof because it was his habit to do so. Or rather, it had been his uncle's habit. Uncle Bilbo had a peculiar fondness for high places, for look-out points and sprawling vistas. And while Frodo hadn't exactly inherited it this love of heights, it was his tradition to spend a few minutes on the roof on every trip. It was more like a moment of silence than anything else, though he'd stopped praying years ago.
The possibility that anyone else might already be up there wasn't a concern, despite the time. He didn't particularly need to be alone during this brief memorial service. But, truthfully, he was half-way through his second drink, so his senses weren't at their sharpest.