According to U.S. officials familiar with the case, al-Qaida had a hand in the abduction. His disappearance was followed by two years of negotiation and then a trade. The Iranian diplomat was swapped for key al-Qaida members, including al-Adel. U.S. intelligence picked up on the group in North Waziristan late last year — they reunited with al-Qaida and took up key leadership roles.
The useful thing about a verb like 'pick up' is that it means many different things - some of which - maybe - are not sources that the authorities want to admit to using. They observed the group, questioned other people, drew conclusions from monitoring news programmes...
Thanks for Your effort! That was a helpful information for me Next time I will keep trying to be more elastic in my translations on the 'pick ups' and other verbs like that.
I think it can be read as "they became aware of" this group. Think of scanning a radio. This is what the intelligence community is supposed to be doing. Then they "picked up" the existence of this group and began surveillance.
Yes, I would definitely say that the " picked up on" in your context that you provided would be that they "tracked" the group or "located their whereabouts".