Now non-monetized YouTube channels also can see ads on some of their videos. By seeing ads some one think that, their channel might monetized. But this is not true , YouTube recently updated Terms of Service to put ads on non YPP (YouTube Partner Program) videos.
YouTube will puts ads on non-monetized channels, but the creators won't receive a share of ad revenue.
YouTube updated its Terms of Service.
Right to Monetize
<i>You grant to YouTube the right to monetize your Content on the Service (and such monetization may include displaying ads on or within Content or charging users a fee for access). This Agreement does not entitle you to any payments. Starting November 18, 2020, any payments you may be entitled to receive from YouTube under any other agreement between you and YouTube (including for example payments? under the YouTube Partner Program, Channel memberships or Super Chat) will be treated as royalties.
If required by law, Google will withhold taxes from such payments.</i>
What this means:
For channels not yet in the YouTube Partner Program:
YouTube added this new section to let you know that, starting today YouTube will begin slowly rolling out ads on a limited number of videos from channels not in YPP. This means as a creator that’s not in YPP, you may see ads on some of your videos. Since you’re not currently in YPP, you won’t receive a share of the revenue from these ads, though you’ll still have the opportunity to apply for YPP as you normally would once you meet the eligibility requirements. You can always check your progress toward eligibility on the monetization tab in YouTube Studio.
You can find more details on Google Terms of Service Update page in below link.
<a href="https://support.google.com/youtube/thread/83733719?hl=en" target="_blank">https://support.google.com/youtube/thread/83733719?hl=en</a>