After everything Fiona's been through, it was only a matter of time before she had a breakdown.
Shameless Season 4 Episode 10 had all of the Gallagher siblings looking for her in fear that she had pulled a Frank.
For the Gallagher kids, is there anything worse than ending up like Frank? Well... ending up like Monica is a close second.
I have to say that the kids have barely given Fiona a second thought throughout Shameless season 4 so far. I'm kind of surprised they noticed or cared she was gone. Even before she screwed up with the coke, Debbie and Carl barely paid attention to her. Lip was off at college. Ian is going through this phase.