Most verbs use either avoir or être as the auxiliary verb in Le Passé Composé(or other compound tense), but rentrer uses both, depending on its grammatical usage* and what it means in the sentence.
*Grammaphile's Corner : the technical grammatical distinction between these cases is actually whether the verb is used in a transitive or intransitive manner. - The transitive version (the version with adirect object) uses avoir. - The intransitive version (lacking a direct object), uses être.
Using avoir or être with the verb RENTRER in Le Passé Composé in French
être + rentré [quelque part] = to go/come in(to) [something/somewhere] = to go/come back in(to) [something/somewhere] = to go/come/get home
Je suis rentréde vacances tard hier soir.I came back from holiday late yesterday evening.
Vous êtes rentrésdans la pharmacie à six heures.You came into the pharmacy at six o'clock.
Quand tu es rentrédans la pièce, elle n'était plus là.When you came back into the room, she was gone.
Note that in each case where êtreis the auxilliary, the verbrentreris followed by a preposition(en, sur, dans, à, etc.). So in these casesrentreris usually aboutgoing or coming back in or into, going or coming home, going or coming in or into.
avoir + rentré [quelque chose] = to take/bring/get [something] back inside
J'ai rentréle linge une fois qu'il était sec.I took the laundry back inside once it was dry.
Vous avez rentréles meubles de jardin à cause de la pluie.You brought the garden furniture inside because of the rain.
When rentreris followed immediately by a noun (as opposed to a preposition), it uses avoiras the auxiliary, like most verbs.
It can be very tricky to get the distinction here if you think in terms of what rentrer means in English. English verbs are very often 'prepositional', meaning we say things like to go back into a house as well as re-enter a house which are equivalent in meaning but grammatically very different - English verbs very often have prepositions where they don't in French!
Here is the list of all "two-auxiliary" verbs in compound tenses: