@Florence he was allegedly carrying $80,000 in cash when he arrived in Moscow (reported by an anti-Putin news outlet and an independent one in Latvia). That's one hell of a wedding gift. The rest of the visiting wedding guests had decided to remain together as a group as a precaution. Whelan left the group, and met a guy in his hotel room, where he was supposedly given a USB stick containing some sensitive material.
@Florence yup, this is not some innocent tourist snatched up as a hostage for a prisoner exchange. If he were working for the CIA or MI-6 he would have had a diplomatic passport. So he was probably working on getting info for BorgWarner that he could sell to the highest bidder.