When using nouns with certain verbs you simply do not use an article. It is the case for motion verbs (a merge, a se duce, a veni) and for the verb „a fi”. Examples:
Du-te la mașină și ia plasele. - Go to the car and take the bags.
Eu vin de la poștă. - I am coming from the post office.
Sunt la universitate. - I am at university.
There might be exceptions, but I was unable to find any, it seems to be a pretty consistent rule.
I believe it's because the noun is placed after a preposition. All nouns coming after a preposition are not articulated. The only exception, I think, is with the preposition "cu", or if the noun has to be articulated, like for the possessive.
Te-am văzut la televizor - I saw you on the TV. (Televizor comes after the preposition "la", and should not be articulated) Merg cu cartea - I walk with the book. (Cartea comes after cu, the only(?) preposition after which you should articulate the noun) Sunt în casa mea - I am in my house (Casa comes after în, but is articulated for the possessive)
I think we learn this in school, but I don't remember the rules anymore. :) ”Cu” seems to be one preposition requiring a noun with article. There are also the (slightly less used) prepositions/prepositional locutions with accusative/genitive/dative din cauza/în spatele/mulțumită, where one has to use an article: din cauza unui băiat/din cauza băiatului - because of a boy/because of the boy.