Engagement levels are unique to your account and recalculated each week based on the number of emails you’ve sent, which contacts you’ve targeted for each mailing, your email open rate, and your email click rate. Based on this, even if a contact had previously not engaged with your material at all, if they've recently started engaging more, then it's highly unlikely that they'll be included in your account's "least engaged" category.
Keep in mind, this is based on the contacts' activity as much as it is yours. Even if you aren't sending as many emails to your high engagement scored contacts, as long as they engage when you do send to them, their score will remain high. So you don't need to worry about a high engagement contact's score deprecating in regards to the weekly recalculations, if you haven't sent them anything recently.
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