Re-re-reading the study spell it is also a utility spell so therefore it can't be used against an unwilling target. This does kind of make it unlikely to be used for nepharious needs - like impersonating someone to commit a crime.
Study is also a spell that ONLY affects the caster, so it being a Utility spell is not an issue at all. It is not cast ON the person studied, it does not touch the "studied person/place/thing" in any manner.
The spell allows for memorization of the target in the level of detail that is required for its use in other spells.
Think of the Study Target spell as being like a snapshot or photo. The target doesn't need to be aware of the picture being taken. The resulting "photo" could be considered to be a "special" memory" that never fades or blurs over time, it is so well burned into the memory. You could even postulate that this special memory might even have a "source tag" (or a special mnemonic) as it were, and that when casting the spell, the spell checks the number of source tags in memory and that is what determines if another can be added.
Freeing up a "slot" then equates to removing the source tag. The memory itself is still there, but it is now affected by the fading and blurring of time.
Another way to look at it would be that the spell takes the memory and put a non-magical post-hypnotic command around it, allowing it to be called up at will, in full detail. Removing a "target" then equates to removing that post-hypnotic command.
As for a person noticing, well the caster IS casting a spell, and that usually is noticeable....
Hope the above makes some sort of sense...