B737-700 Flight Attendant reported being notified through a report that a passenger's unauthorized battery powered bag was transported in the passenger cabin.

Narrative

During the boarding process at ZZZ; a customer boarded the flight at the last moment. At the time; I was making on-board announcements and also facilitating other passengers' use of the lavatory. To expedite boarding; I took my dedicated assist position at the entry door to allow room for a late-arriving passengers.Using my peripheral vision; the passenger's bag appeared to comply with size requirements for carryon items; fitting into overhead bins. Consequently; I did not inspect the item closely; as my focus was on completing the boarding process as quickly and efficiently as possible.After landing and while addressing a separate customer complaint; which had been previously reported through a report; a Company Agent informed me that the late-arriving passenger's bag" was actually an unauthorized battery-powered bag.Ground staff should be reminded of the policies regarding unusual or potentially hazardous carry-on items and should be empowered to make decisive actions accordingly. Reinforce the importance of checking the dimensions and nature of all carry-on items before passengers are allowed to board."