I've played plenty of solo rounds at Seatac and never felt threatened. During the day, the sketchy looking people are usually the disc golfers.
Being a big public park next to an airport, you're bound to see some "elements" now and then, but I've never had any trouble. Once a drunk and/or high guy was crossing a fairway, he watched me drive, and then as he sauntered past my disc, he picked it up and started walking away with it. I yelled at him, and he casually threw it on the ground and kept walking. That's my horror story from Seatac.
Like cefire said, just be careful when you park and keep valuables out of sight. Break ins have happened both on the street and in the parking lot.
Oh, and I would venture to guess that playing glow golf wouldn't be the best idea there either.