Actually, your salary from days worked in the US is considered US income!
It doesn’t matter where you were paid or where your employer is located. It only matters where you were when you conducted the work. Days in the US for work, training, work conferences, and consulting usually result in US “effectively connected income.”
This income usually needs to be reported on a US tax return. And on certain state tax returns, too! Fortunately, in cases where the income amounts are small, an exception is available. Also, some countries have tax treaties with the US that enable you to avoid US tax on this income.