Tracking Someone 4th ed

[HMS Invincible]

How would you track someone down who doesn't want to be found? A exPC 9+ lvl ranger has instructions to screw with us and keep leveling. He has stolen a semi-mcguffan from us and some party loot. He has a equal level 2 handed weapon fighter exPC with overpowered stats(all 18s) The ranger has the enchant magic item ritual + w/e magic items the DM wants to make the story appropriate or lets him screw with us and w/e the DM feels appropriate to make the encounter hard. For example, he gave the Ranger a hat of disguise so he could mug our stupid bard in our party.
How should I deal with this? The hardest way is to scry and die the fellow but I know that leads to potential for all sorts of DM bullshetiry.

I am a 10th lvl wizard with a hack and slash party. The best they can manage is I point them in the right direction and they smash anything that fights back.
E.g. I send our party rogue who's background is a trained assassin. He goes to someone in town and asks "Have you seen this young ranger?"
"He has already left town."
He comes back to report "I cannot track down this ranger".

It's been a couple levels since this has happened, mainly when I wasn't attending the session.
Now our one bungled attempt at finding him was wasted, and we lost a month in game time. This mcguffan was intended to help stop a war, but I had to call it quits because we were losing too many assets without any leads.

However, I do want to deal with this eventually because:
1. Roleplaying wise, I can't account for inflation which says we gain more treasure from encountering high level stuff then we do from retrieving low level treasure. The more time passes, the less the stolen goods are worth. I should just ignore him and concentrate on more rewarding adventures, but that would be metagaming.
2. My party's failure/humiliation is my failure because I am the party leader, and this is now personal.

Should I hire a tracker(aka have the DM help me), or someone who can trawl the latest rumors?
Should I try to track him myself? What are good questions to ask?
Should I rely on magic? The ranger has conflicting goals, to hide from us, and to continue adventuring/leveling.

Whatever I do, I have to do it in a way to satisfy my DM's urge that I have an "adventure" instead of killing this guy and moving on to the actual plot. It can't be too smart else he'll pull Shroedinger's cat on me and say, "well, he somehow managed to think about and counter your plan."

[Tempest Fennac]

If he has a Hat of Disguise, finding him without magic could be next to impossible. I think you should try scrying for him if you can.