In every Omniconvert experiment, there are 3 goals that come by default: engagement, bounce rate and sales.

An engagement goal is counted when a user, after he was included in an experiment, has visited more than 3 pages on your website and stayed  more than 180 seconds.

The engagement cookie has a timeout set to 1h, time in which the engagement conversion must happen.

This means that if an user lands on a website page where an experiment is running, then goes on to another page after which leaves the website and returns after an hour or more, the engagement conversion will not register.