Detectably is an adverb that refers to the quality of being perceptible or noticeable. The antonyms for detectably would therefore be words that denote the opposite of this quality, such as indetectably, imperceptibly, or unnoticeably. These words suggest that something is not easily perceived, distinguished, or detected. For example, an indetectable flaw in a product may not be noticeable to the naked eye, while an imperceptible change in temperature may not register on a thermometer. In contrast, something that is detectable is easily discernible or recognizable, as if it were within our reach.