Yes, it could be. But it could be other reasons as well; it's hard to say specifically. Here's some standard IRS language regarding the CP05 notice:
We sent you this notice because we’re reviewing your tax return to verify:
- The income you reported on your return
- The income tax withholding amounts you reported on your return
- The claims for tax credits you made on your return
- The withholding claimed on your Social Security benefits
- The household help you claimed
- The Schedule C income you claimed
Normally with CP05 notices, you simply wait until they conclude their initial review and send you a letter. They may request additional info. They should've given you some sort of timeframe in the notice. In the meantime, you should assemble your documents and re-familiarize yourself with the return.
