Is there an S Corp asset that is other than 100% for business? What is the asset and the nonbusiness user of it?

There is a computer that is used only partially for business.

Then it shouldn't be in an 1120S entity.  Schedule C type entities can have mixed usage, but not corporations

It is mixed use and it is an S-Corp. Just as vehicles could be mixed use, so too can other assets.

It you have mixed use vehicles, that personal use gets added to the person's W-2.  Hopefully you are following the rules and handling it that way.

Where to put?

Nowhere, unless you consider  W-2 as the place to put it.

"It is mixed use and it is an S-Corp. Just as vehicles could be mixed use, so too can other assets."

What you just described there is personal use of corporate asset. Not "mixed" use. Personal use of corporate asset typically is taxable to the person benefiting. There is no % partial or mixed use. There is 100% S Corp owned and operated, and then taxable value to someone who used it personally. Think of it as a type of cost recapture, because the corporation can't partially own an asset or only expense a part of it. It doesn't share, in other words.

Is this a home office environment, and someone bought a computer "for the corporation" and it's in the home office?