Think of

garder trace de…

as a whole.
“de” would be used in a sentence like:

on n’en a pas gardé de trace.

There are lots and lots of combinations of verb + noun without an article.

• To have and common verbs can be combined with a lot of nouns: avoir envie, avoir faim, prendre acte, prendre froid, donner congé, donner raison, …

• It also happens with specific verbs and nouns.

Sometimes the meaning is evident, sometimes, it is specific. It is impossible to know them all. They are in dictionaries.

• garder trace = garder une trace

• ne pas garder trace = ne pas garder de trace