In most systems of magic that I'm familiar with such a spell would be of an intermediate or difficult variety, so we can eliminate any novices and apprentices who might be here. Unfortunately, I can count on one hand the number of people I'm familiar with who might be capable of such magic and are currently in the City, and none of them are likely suspects.

So, the perpetrator is likely to either be someone relatively new or very secretive, perhaps both.

This is assuming that whoever it is wasn't using an invisibility cloak or something, but again - someone would have to have manufactured that, and I don't know that anyone here is in the business of magical items.