In some cases it maight be some third-party service trying to send POST requests to your website.
You can check the othe info about these requests like Event URL and User agent - these should help you to identify what is this.
For example, I've seen this in my event log on one of the websites:
Event URL: /autodiscover/autodiscover.xml
User agent: ExchangeServicesClient/15.00.0913.015
It's more likely some mail server/client trying to sent POST request to the website. I'd say there is basically nothing to worry about.