adverb
before one knows it; without realizing; unnoticed
Describes a change or event that happens so gradually or subtly that the speaker does not notice it happening. Similar to いつの間にか, but いつの間にやら has a slightly more colloquial, old-fashioned, or literary feel.
See also: いつの間にか
いつの間にやら外は暗くなっていた。
Before I knew it, it had gotten dark outside.
いつの間にやら彼は部屋からいなくなっていた。
Without my noticing, he had left the room.
いつの間にか is the more common, neutral equivalent. いつの間にやら is slightly more colloquial or literary, and may carry a nuance of surprise or old-fashioned flavor.
Derived from いつの間にか (itsu no ma ni ka) with the colloquial/emphatic particle やら replacing か. The exact historical development is uncertain, but やら adds a nuance of uncertainty or exclamation.