Why TDS being deducted did not settle it
The common assumption is that once tax is deducted at source, the obligation is over. For an NRI that is only true in a narrow case. Under Section 115A(5), you are excused from filing a return only if your total income consisted solely of dividend or interest (and certain royalty or technical-fee income) on which tax was deducted at the rate the section prescribes or higher. That box is small.
The moment you also had rent, a capital gain from a property or mutual-fund sale, or any other head of income, you fall outside the carve-out and a return was due. And even within the carve-out, if the tax was over-deducted, the only way to recover the excess is to file. So TDS does not close the matter for most NRIs; it is the starting point, and the notice is the department pointing out the gap.
What the notice is, and why you answer it
The notice usually comes as a Section 142(1) requirement to furnish the return, backed by the non-filer e-campaign that the department runs off the income and TDS visible against your PAN (Section 142(1)). It is not yet an assessment; it is a direction to file, and responding is straightforward: file the pending return, report the income correctly, claim the TDS, and where it was over-deducted, claim the refund.
For many NRIs in this position, filing is actually to their advantage, because the TDS deducted on rent or a property sale is usually far more than the real tax, so the return produces a refund rather than a demand. The notice, in other words, is often the prompt that gets you money back, not a bill.
What happens if you keep not filing
Ignoring the notice is where it turns costly. If you continue not to file, the officer can make a best-judgment assessment under Section 144, estimating your income from the material available, which will not credit the deductions and exemptions you never claimed, so the figure is usually worse than your real position. On top of that come interest under Sections 234A, 234B and 234C, and a penalty for under-reporting under Section 270A, with prosecution possible in serious cases.
So the choice is between filing correctly, which for most NRIs in this spot yields a refund, and leaving it to a best-judgment assessment that assumes the worst. A practising CA works out whether a return was actually due, files the pending years, claims the TDS and any refund, and responds to the notice, all remotely under your authorisation.