["Mission failure imminent: eliminate remaining targets"]

So, me and my friend started a sabotage mission on uranus and we were going to find 3 Grineer caches that we needed for neptune junction. Since those caches are very well hidden and the map was huge we were in for 30mins and we had 1 more cache to find. he said he had to go for like 10 mins and maybe 2 mins after he went afk I've got this "Mission failure imminent: eliminate remaining targets". I had no idea were those enemies were and i was around 1600m from extraction point, where my friend was afk. And having the 1 minute countdown ran out we failed the mission and our 30~35 mins were completely wasted. Can anyone explain why did that happend?