We've all changed our behavior to deal with what we fear is coming -- a teenager stops to get gum to cover up the smell of alcohol that his parents will be looking for when he comes home, for example. If an IC is separated from the MC for an extended period, can the MC's imagination ("I know he's out there doing X.") fill the role of the IC? Or must there be a hand-off?
Yes, an absent IC can affect the MC. However, as an author communicating to an audience, it is important that you make the connection between the IC and MC even during the absence.
In your example above, you talk about a teenager getting gum to cover up alcohol breath in order to hide it from his parents. Showing or describing a teenager getting gum to cover up alcohol breath is insufficient. SOMEHOW, you must indicate to the audience that the teenager is doing this BECAUSE he wants "to hide it from his parents." Without that connection, we cannot see the influence the parents have on their teenager.
There are many techniques for conveying the position or perspective of a character in its absence. Objects belonging to the character, character mementos or pictures -- especially pictures of the MC and IC together -- can work well to let the audience know about the impact the IC has on the MC. Memories of the IC doing something or saying something are popular, if not overused. You can also bring it up the IC in dialogue with other characters who say something like, "You know what [IC] would say about that!" or something more subtle. Yet another way is to show symbols of important events in the Relationship throughline (MC/IC), or have a third party use an IC phrase or an item symbolic of the IC. In each case, these symbols, phrases, places, or events must be set up ahead of time so the audience can attribute them properly to the IC.
In extreme cases, where the IC is gone for an Act or more, you may need to hand off the function to another character. This is particularly true if your story emphasizes Character above Plot, Theme and Genre. The original Star Wars got away with having Obi-Wan (IC) gone for nearly half of the story because Character was emphasized less than Genre and Plot, and about equal with Theme development. As it is, Act III has Luke remembering Obi-Wan and missing him -- for about fifteen seconds, and then Obi-Wan's disembodied voice comes in toward the end of Act IV to give Luke one last push, "Use the Force, Luke. Let yourself go."
Most stories, with Impact Characters gone that much, hand off the job to a different character. But you don't HAVE to.