Clarifying that I tried monster form and animal form and they were still calling guards immediately. It didn't change their behavior.
Tracking has a 10 second cooldown and monsters, animals, and players are three separate checks and it's not 100% detection rate. Sure they could be tracking at all times and then using the distance of the arrow from their characters to enact a detection radius, but I would think they would need a ridiculous amount of accounts to have tracking coverage of all three forms at the rate they were displaying. Another detail is that each character had separate zones of detection, i.e., i could trigger one character and the others would stay silent. Still doable with information sharing between clients, but just makes it even more of a stretch as all of this requires so much effort for a guards script that can be done much more simply (and much less obviously) just by using journal scan. I don't get it.
I vaguely recall methods of inference using Crystal Ball of Knowledge and Detect Hidden skill to glean nearby characters by the difficulty message (I might be conflating this with something else), but they did not have anything like that in their packs. Maybe there's a similar method? In their packs they had runebooks, atlases, house placing tools, interior decorator tool, mastery book, ethies, luck statues, a couple of them had pirating tools (torch, ramrod, swab), green thorns for the event, and a trapbox that they were putting eggs and loot into. Their suits were standard luck suits, only item with any sort of potential interaction was spell focusing sash which they were all wearing. All three were tamers with 2 cus and a fire steed.
This has intrigued me so much I even double checked to make sure clients don't receive packets from the server with information about nearby stealthers and I can confirm they do not. At least passively, I dunno if there is a call and response packet interaction possible and I'm not educated enough on UO packet encryption to make much headway on that theory.
Anyway, next time I see 'em, I'll bring my own tracker to check for additional characters and also try running straight at them from off screen as I might've been giving them enough time to catch me on tracking.