"El espejo sigue aquí."
When pasting this into Google Translate, it gives "The mirror still here." It seems to translate "sigue aquí" to "still here" as a unit. It seems though, that if that were a correct translation, we would need a helper verb, a la "el espejo está sigue aquí."
When putting "The mirror is still here" or "The mirror remains here" into GT, both give this phrase as a correct translation. "is still here" or "remains here" both produce "sigue aquí" as options.
However, "sigue" can mean "continues" or "still" so it appears that's how it's being used here: "The mirror continues [to be] here" or "The mirror [is] still here." I'm still not sure why either ser or estar is not needed though.
I talked to my sister (fluent) and her husband (native) and they say the best way to say this would be "el espojo se queda aquí." Neither seems to be sure whether duolingo's translation is acceptable. They did say that using estar in the phrase "sigue estar aquí" would not work though. Either this is wrong, or it's just one of those cases where two words mean something different together than each means alone.