No, you should not be worried about an audit if still processing after 21 days.
The dates for receiving a tax refund after it has been accepted by the IRS are only estimates based on the IRS target of issuing a tax refund after 21 days from the date it was accepted by the IRS. This is a target only and not a guarantee.
After the tax return has been Accepted by the IRS (meaning only that they received the return) it will be in the Processing mode until the tax refund has been Approved and then an Issue Date will be available on the IRS website.
See this IRS website for federal tax refund FAQ's - https://www.irs.gov/refunds/tax-season-refund-frequently-asked-questions
If over 21 days since being Accepted by the IRS and the tax refund is still Processing you can call the IRS and speak with an IRS agent concerning your tax refund.
Call the IRS: 1-800-829-1040 hours 7 AM - 7 PM local time Monday-Friday
@jorhawk I’m in the same boat. It’s been 25 days and I only have the still processing message.