It probably works how the caster wants or expects it to work.
When she spies on Marco and Jackie, it follows them at a fixed distance and they're totally unaware of it. They might just have not noticed, but a spying spell that let people know you were spying on them isn't very useful.
When she tries to spy on Glossaryck, she has to search for him, and then it appears as a portal in space on both ends. However, she had expected to be able to just reach through and grab Glossaryck when she casted the spell. Then she completely reworked the spell on the fly to let her do it, after Glossaryck told her it couldn't be done.
Basically we should just assume Star's magic does whatever she wants it to, if she's focused enough on doing it.