If you were just trying to track or scry on the gum (because you knew it would lead to the target as well), would you need a picture of the target? Would that lower the complexity of the tracking spell, since you wouldn't have to overcome the gum's Conviction? (This assumes that you didn't use a truly righteous gum, of course...)
Consider that as far as DFRPG is concerned, what you're really doing is using magic as a substitute for an Investigation roll. Being magic, it makes impossible things possible, etc. But even given all that, and assuming it was possible to locate a person or piece of gum in a city ... which would be easier? I'd tend to say that even if it was possible to locate gum somewhere in the city, it would be in the legendary range. So I guess I see it still as trying to find the target, but with the gum providing the necessary link. The divided gum then allows you to make a declaration involving the law of contagion, and the picture allows you to make a declaration involving the law of similarity; the two aspects you create can then be tagged to either fulfill the complexity requirement (useful if you want to bump up the complexity to deal with potential wards or make the spell work faster) or to aid in controlling the spell (useful to prevent flubbing the spell). By the way, the whole similarity/contagion thing assumes your spellcaster believes in that style of magic; a Shaman would do things differently.
By the way, I just checked the rules on symbolic links, and they indicate that even a sculpture of the target would be enough to meet the minimum necessary requirements of casting Thaumaturgy. So if your character had a bit of an artistic bent and a few photos to work with, he might be able to craft a suitable symbolic link without even being in the target's presence. That said, my assumption is that such a link would provide no bonuses in and of itself.
Fair enough. I was hoping that a decent "marking" spell would be low enough complexity that it could be contained inside an enchanted item - that would eliminate the need for chalk circles, etc. For example, an enchanted bullet that fires like a normal bullet, but also contains a hidden spell payload. So even though your bullet just bounced off the Supernaturally Tough skin of the Ogre you shot at, what he doesn't realize is that you've marked him now.
I think that you could use the enchanted item idea to give yourself an aspect to tag for a bonus, but it wouldn't be a 'symbolic link'. At least, not the way you worded it. DFRPG is very subjective and dependant on the story you tell, though, so maybe it would work. Or alternatively, you could describe the bullet enchantment as being the magical equivalent of a leaky paint bottle attached to the target, leaving a trail behind him. The possible downside is that there's only so much 'paint' the bullet can hold, so to speak, and anyone with the Sight would be able to see the spell's effects (thus possibly tipping off your quarry).
Good point. What about spiking another magical potion? "Here's the Potion of Inhuman Speed that you asked for. What's that? It's tastes funny? It's a magic potion - it's supposed to taste funny!" On the other hand, I have no idea what effect mixing potions together would have...
I think mixing potions is probably a very, very bad idea. I think the best case scenario is that the resulting mixture becomes a mundane, foul-tasting drink. Worst case scenarios probably involve explosive reactions.
Keep in mind, by the way, that all of these systems are extremely subjective, and these are my opinions. Your mileage may vary. The most important things to consider are (a) fun, and (b) game balance -- and remember that anything you convince your GM to let you do is more than likely to be done back to you at an inconvenient time. So it's possible that undetectable symbolic-link-aspect evocations capable of guiding TMDs (Thaumaturgies of Mass Destruction) are not a good idea.
