"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?