The PCs are around 10th level right now. There are four in the party. I'd kind of like to have the Zhentarim track and follow them if they leave the city, but I want it to be believable. The Zhentarim has access to a lot of magic and resources, but it is also a bureaucracy and a trade organization. It doesn't waste money. Any thoughts on how I could realistically harass them on the roads leading away from the Keep? I mean, I could throw in some random encounters with caravans and so forth, but I think a scout patrol or something where they are specifically looking for these guys would be best. The only problem: the party left no witnesses, not even a corpse to Speak with Dead.
Not a problem. First though, let me congratulate you: player-driven campaigns are the most fun of all when you've got the right group and it sounds like you're doing a great job for them

I'd also like to give you props for not giving into temptation and fudging the die roll for the bad guy's save!
How do the Zhentarim know to look for them? Easy.
Three words: Contact Other Plane
Two words: Stone Tell
One word: Commune
Another word: Divination
Another word (if they want to bust out the big resources): Hindsight (CAdv)
Several clerics playing Twenty Questions could quickly arrive at the fact that Marcus is dead, where he died, how many people killed him and quite possibly other details. Depending on how you rule the perceptions of a stone, an allied 11th-level Druid casting
stone tell in the room where the party got him will reveal whatever someone standing in the room would have seen.
Hindsight - if you're willing to allow them access to a caster of sufficient level (16th-level Bard, 17th-level Wizard), or simply a scroll of the spell in case of emergencies - will reveal
exactly what happened and who did it.
Alternatively, maybe someone in the Zhentarim has managed to resurrect the old 2nd-Edition spell "Time Pool" (Tome of Magic, Cleric 5) that allows you to look back in time for a specific event. Naturally they'd try to keep this a closely-guarded secret.
Basically, there are tons of ways the party can get rumbled: perfectly legitimate, well within the resources of the Zhentarim and without any DM-railroading. They might not have exact descriptions, depending on the method you decided on, but Twenty Questions could easily get number of people in the party, their races, their classes, whether or not they're still in the city, whether or not any of them are already known to living members of the Zhentarim and so on, and so forth.
If you don't *want* Marcus to be dead, perhaps he had a contingent spell (or magical item) in effect that slices off a toe and teleports it to a safe location in the event of the bearer's demise? The party
did check the body thoroughly before stuffing it into the Bag of Holding, didn't they? No?
That wouldn't be an unreasonable precaution for a powerful organisation to take with its ranking staff, and means it's only a basic
resurrection to get him back. 10,000 is better than 25,000!
Still, if it were me, I'd probably let the party have succeeded in their plan without any ret-cons... as I say, there are plenty of ways in which the Zhentarim can find out how the guy was killed without recourse to any shenanigans whatsoever.
As a side note,
speak with dead wouldn't help anyway: he died in his sleep so his corpse wouldn't know what had happened
