What's he hiding from? The thing in the underlevels that escaped its containment field again? Government agents asking about the UFO that's been spotting flying overhead? The surprise birthday party that Sully organized (it's just him and the government agents)?

Telegram sticker by  arkylarky

Let's see, what's a miscellaneous fact for this one...

Emmet's scarf doesn't "mean" anything in particular to him. It's just that when you work with machines you can never have enough cloth rags, and your neck is as convenient a place to keep a spare one as any.

It was Sully who interpreted a red bandana as part of the Wombat For Hire "uniform" and adopted it as his badge of office, of sorts. When he is seen without it, it is either before he started working for Emmet or after he stopped.

(Sully's sartorial interests go beyond accessories. Hidden in a drawer at Sully's workbench is a design for matching jumpsuits with company logo patches. Thus far, the ferret has had enough sense not to show that to Emmet.)