Another way would be to be “pulled” from a sampling group. There are a set amount of returns that are randomly pulled and audited for different issues. There is no specific reason the return is questioned, it simply fell into the sample group.
If the income you report on your return, does not match the income provided to the IRS via W-2s, 1099, etc., then an “IRP” report is generated and your return is questioned. These types of inquiries are usually handled through correspondence via mail and in most cases are easily resolved.
All returns are subject to the statute of limitation. If an individual return is not audited within three years of its filing date, the return will not be audited; however, if there is reason to believe the return is based on fraud, the return is subject to audit at any time.