auto enrollment for rehires who opted out previously

Auto enrollment is set to 3%.

Participant who had opted out and never deferred is rehired.

I can't seem to find this in the Plan Doc.

Would they need to opt out again, or would the original opt out still stand? Not sure to what extend number of breaks in service would affect this.

I'm guessing that perhaps it is in an administrative procedures addendum, if your document has one? If it truly isn't addressed anywhere, I would just make sure they opt out again. No harm, no foul - even if it is hidden in the document so well that it is impossible to find, can't hurt to have them opt out again, even if isn't strictly necessary.

You may want to check with the author of the plan document for clarification.  Plus the answer may depend on if this is an EACA, QACA, etc.   For instance, Treas. Reg § 1.401(k)-3(j)(2)(iv) regarding QACAs states: 

(iv) Treatment of periods without default contributions. The minimum percentages described in paragraph (j)(2)(ii) of this section are based on the date the initial period begins, regardless of whether the employee is eligible to make elective contributions under the plan after that date. However, for purposes of determining the date the initial period described in paragraph (j)(2)(ii)(A) of this section begins, a plan is permitted to treat an employee who for an entire plan year did not have contributions made pursuant to a default election under the qualified automatic contribution arrangement as if the employee had not had such contributions made for any prior plan year as well.

Here, the plan is permitted to provide for something... but it doesn't have to....