How can the congregation know whether a hymn is also a prayer after Mass?

I recently heard that at the end of Mass, if the recessional hymn has the form of a prayer that the priest is supposed to remain at the altar until the hymn/prayer is done. I'd imagine this is coordinated between the priest and the music director so that the priest isn't taken by surprise and has to lasso the altar boys. Is there a list of hymns which are also prayers or some other cryptic markings in music that indicate the hymn is also a prayer. This was a hymn on the Feast of the Archangels, where the three verses were prayers specific addressed to each of the archangels. I believe it was this hymn: > Christ, the fair glory of the holy angels, Thou Who hast made us, Thou > Who o’er us rulest, Grant of Thy mercy unto us Thy servants Steps up > to Heaven. > > Send Thy archangel, Michael, to our succor; Peacemaker blessèd, may he > banish from us Striving and hatred, so that for the peaceful All > things may prosper. > > Send Thy archangel, Gabriel, the mighty; Herald of Heaven, may he from > us mortals Spurn the old serpent, watching o’er the temples Where Thou > art worshipped. > > Send thy archangel, Raphael, the restorer Of the misguided ways of men > who wander, Who at Thy bidding strengthens soul and body With Thine > anointing. > https://hymnary.org/text/christ_the_fair_glory_of_the_holy_ang_th Which definitely reads more like a prayer than "How Great Thou Art" or "Morning has Broken" Usually the recessional hymns are just the normal songs from the hymnal, but I think this one was special in some way - possibly specific to this feast day?